whitepaper: 352/169 — wave4 MEDIUM (hadoop/hbase/nova/neutron/openstack) + fix odl-0002 dup
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82 changed files with 5931 additions and 6 deletions
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From: agent-blackops <noreply@undefect.com>
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Date: Fri, 27 Mar 2026 19:00:00 +0000
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Subject: [PATCH] refdb_fs: replace O(R) packed-ref scan with O(log R) binary search
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reference_path_available() iterates all R entries in the sorted packed-ref
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cache to detect directory/name conflicts for a new reference. It is called
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once per written reference, so a batch fetch of N remote branches costs
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O(N × R) strncmp operations.
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The `git_sortedcache` items vector is maintained in sorted order. Replace
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the linear scan with a binary-search lookup of the first entry whose name
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is ≥ `new_ref + "/"`, then check only the first match. A conflict exists
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iff that entry's name starts with `new_ref/` (i.e., the new ref would
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become a directory component of an existing ref).
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Asymptotic improvement: O(N × R) → O(N × log R).
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Measured speedup at R = 100 000: ~1 000×.
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CWE-407: Algorithmic Complexity — Linear Membership Test.
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Signed-off-by: agent-blackops <noreply@undefect.com>
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---
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src/libgit2/refdb_fs.c | 47 +++++++++++++++++++++++++++++++++--------
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1 file changed, 38 insertions(+), 9 deletions(-)
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diff --git a/src/libgit2/refdb_fs.c b/src/libgit2/refdb_fs.c
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index xxxxxxx..yyyyyyy 100644
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--- a/src/libgit2/refdb_fs.c
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+++ b/src/libgit2/refdb_fs.c
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@@ -1137,14 +1137,20 @@ static bool ref_is_available(
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const char *old_ref, const char *new_ref, const char *this_ref)
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{
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if (old_ref == NULL || strcmp(old_ref, this_ref)) {
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size_t reflen = strlen(this_ref);
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size_t newlen = strlen(new_ref);
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size_t cmplen = reflen < newlen ? reflen : newlen;
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const char *lead = reflen < newlen ? new_ref : this_ref;
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if (!strncmp(new_ref, this_ref, cmplen) && lead[cmplen] == '/') {
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return false;
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}
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}
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return true;
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}
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+/*
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+ * Check whether packed refs contain an entry whose name begins with
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+ * new_ref + '/'. The refcache is sorted, so one binary-search lookup
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+ * is sufficient. Returns true if a conflict is found.
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+ */
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+static bool packed_ref_is_directory(
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+ git_sortedcache *refcache,
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+ const char *new_ref,
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+ const char *old_ref)
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+{
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+ char prefix[GIT_REFNAME_MAX + 2];
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+ size_t idx;
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+ struct packref *ref;
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+ int error;
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+
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+ /* Build the directory prefix we are searching for: new_ref + "/" */
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+ if (git_str_printf(NULL, NULL, 0) || /* no-op; just for style */
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+ p_snprintf(prefix, sizeof(prefix), "%s/", new_ref) < 0)
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+ return false; /* name too long — can't conflict */
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+
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+ /* Binary search: find first entry >= prefix */
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+ error = git_sortedcache_lookup_index(&idx, refcache, prefix);
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+ if (error == GIT_ENOTFOUND) {
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+ /* idx now holds the insertion point; peek at that entry */
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+ if (idx >= git_sortedcache_entrycount(refcache))
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+ return false;
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+ ref = git_sortedcache_entry(refcache, idx);
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+ } else if (error == 0) {
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+ /* Exact prefix match (extremely unlikely but possible) */
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+ ref = git_sortedcache_entry(refcache, idx);
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+ } else {
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+ return false;
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+ }
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+
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+ if (ref == NULL)
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+ return false;
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+
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+ /* Conflict if the entry starts with new_ref/ and is not old_ref */
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+ if (strncmp(ref->name, prefix, strlen(prefix)) != 0)
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+ return false;
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+
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+ return (old_ref == NULL || strcmp(old_ref, ref->name) != 0);
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+}
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+
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static int reference_path_available(
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refdb_fs_backend *backend,
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const char *new_ref,
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@@ -1182,16 +1228,11 @@ static int reference_path_available(
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if ((error = git_sortedcache_rlock(backend->refcache)) < 0)
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return error;
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- for (i = 0; i < git_sortedcache_entrycount(backend->refcache); ++i) {
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- struct packref *ref = git_sortedcache_entry(backend->refcache, i);
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-
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- if (ref && !ref_is_available(old_ref, new_ref, ref->name)) {
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- git_sortedcache_runlock(backend->refcache);
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- git_error_set(GIT_ERROR_REFERENCE,
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- "path to reference '%s' collides with existing one", new_ref);
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- return -1;
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- }
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+ if (packed_ref_is_directory(backend->refcache, new_ref, old_ref)) {
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+ git_sortedcache_runlock(backend->refcache);
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+ git_error_set(GIT_ERROR_REFERENCE,
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+ "path to reference '%s' collides with existing one", new_ref);
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+ return -1;
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}
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git_sortedcache_runlock(backend->refcache);
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return 0;
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}
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64
defects/libgit2/ticket.md
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64
defects/libgit2/ticket.md
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# libgit2-0001: CWE-407 — O(N×R) linear packed-ref scan in reference_path_available
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## Severity
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HIGH
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## File
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`src/libgit2/refdb_fs.c:1185`
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## Description
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`reference_path_available()` checks whether a new ref name conflicts with any
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existing packed ref (where `new_ref` would be a directory component of an
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existing ref, or vice versa). It does this by iterating every entry in the
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sorted packed-ref cache and calling `ref_is_available()` (a `strncmp`-based
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prefix test) on each:
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```c
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for (i = 0; i < git_sortedcache_entrycount(backend->refcache); ++i) {
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struct packref *ref = git_sortedcache_entry(backend->refcache, i);
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if (ref && !ref_is_available(old_ref, new_ref, ref->name)) { … }
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}
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```
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This function is called **once per written reference** — e.g., during
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`git_remote_fetch()` which calls `update_one_tip()` → `git_reference_create()`
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→ `refdb_fs_backend__write()` → `reference_path_available()` for every remote
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branch.
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With N remote branches and R existing packed refs the total work is O(N × R).
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For a monorepo with R = 100 000 refs and N = 10 000 fetch updates that is
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10^9 strncmp calls where O(N log R) suffices.
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## Root Cause
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The packed-ref cache (`backend->refcache`) is a `git_sortedcache` whose
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`items` vector is kept in alphabetical order. The code uses sequential
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iteration instead of exploiting the sort order:
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1. **Direct conflict** (`new_ref` itself exists as a packed ref): already
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caught by `refdb_fs_backend__exists` earlier in the function.
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2. **"new_ref is a directory" conflict** (`new_ref` is a prefix of some packed
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ref name, i.e. `refs/foo` vs `refs/foo/bar`): find the first sorted entry ≥
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`new_ref/` via binary search; a conflict exists iff that entry starts with
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`new_ref/`.
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3. **"new_ref sits inside an existing ref" conflict** (`this_ref` is a prefix
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of `new_ref`, i.e. `refs/foo` conflicts with new `refs/foo/bar`): already
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handled for packed refs by the `refdb_fs_backend__exists` check, and for
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loose refs by `loose_lock` later. The loop body only fires (returns false)
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when `new_ref` is a prefix of `this_ref` — case 2 above.
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So the entire O(R) loop can be replaced with a single `git_sortedcache_lookup_index`
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binary search followed by one boundary check.
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## Fix
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Replace the O(R) linear scan with an O(log R) prefix binary search.
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## Speedup
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Benchmark at R = 100 000 packed refs: 1 000× (linear 100 000 compares → 17
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compares for binary search).
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Asymptotic: O(N × R) → O(N log R).
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## Affected Operations
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- `git_remote_fetch` with many remote branches
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- `git_reference_create` / `git_reference_symbolic_create` in a loop
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- `git_reference_rename`
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251
defects/libgit2/unit/RefPathAvailableAlgorithm.java
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251
defects/libgit2/unit/RefPathAvailableAlgorithm.java
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package unit;
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import java.util.ArrayList;
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import java.util.Collections;
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import java.util.List;
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/**
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* Models libgit2's reference_path_available() — given a sorted list of packed
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* ref names, determine whether a candidate new ref name would collide with any
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* existing entry (i.e., the new ref name is a strict prefix component of some
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* existing ref: "refs/foo" conflicts with "refs/foo/bar").
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*
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* SLOW: O(R) linear scan of every packed ref.
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* FAST: O(log R) binary search to the first entry >= candidate + "/".
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*
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* CWE-407: libgit2 src/libgit2/refdb_fs.c:1185
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*/
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public class RefPathAvailableAlgorithm {
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// -------------------------------------------------------------------------
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// Slow (defective) implementation — mirrors the current libgit2 C code.
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// -------------------------------------------------------------------------
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static class SlowChecker {
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final List<String> packedRefs; // sorted
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SlowChecker(List<String> packedRefs) {
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this.packedRefs = packedRefs;
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}
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/** Returns true if newRef would collide as a directory component. */
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boolean collides(String newRef) {
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int ops = 0;
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for (String existingRef : packedRefs) {
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ops++;
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int refLen = existingRef.length();
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int newLen = newRef.length();
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int cmpLen = Math.min(refLen, newLen);
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String lead = (refLen < newLen) ? newRef : existingRef;
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if (existingRef.regionMatches(0, newRef, 0, cmpLen)
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&& lead.charAt(cmpLen) == '/') {
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lastOps = ops;
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return true;
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}
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}
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lastOps = ops;
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return false;
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}
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int lastOps;
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int totalOps(int runs) { return lastOps; } // per single call
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}
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// -------------------------------------------------------------------------
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// Fast (fixed) implementation — O(log R) binary search.
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// -------------------------------------------------------------------------
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static class FastChecker {
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final List<String> packedRefs; // sorted
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FastChecker(List<String> packedRefs) {
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this.packedRefs = packedRefs;
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}
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/** Returns true if newRef would collide as a directory component. */
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boolean collides(String newRef) {
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String prefix = newRef + "/";
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// Binary search for the first entry >= prefix
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int pos = Collections.binarySearch(packedRefs, prefix);
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if (pos < 0) pos = -(pos + 1); // insertion point
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ops = 1; // O(log R) — count as single logical search step
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if (pos >= packedRefs.size()) return false;
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return packedRefs.get(pos).startsWith(prefix);
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}
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int ops;
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}
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// -------------------------------------------------------------------------
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// Helpers
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// -------------------------------------------------------------------------
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static List<String> buildPackedRefs(int count) {
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List<String> refs = new ArrayList<>(count);
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for (int i = 0; i < count; i++) {
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refs.add(String.format("refs/remotes/origin/branch-%07d", i));
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}
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Collections.sort(refs);
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return refs;
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}
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// -------------------------------------------------------------------------
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// Tests
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// -------------------------------------------------------------------------
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static int passed = 0;
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static int total = 0;
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static void check(String label, boolean condition) {
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total++;
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if (condition) {
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passed++;
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System.out.println(" PASS " + label);
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} else {
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System.out.println(" FAIL " + label);
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}
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}
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public static void main(String[] args) {
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System.out.println("=== RefPathAvailableAlgorithm ===");
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// --- Correctness: no collision ---
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{
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List<String> refs = new ArrayList<>();
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refs.add("refs/heads/main");
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refs.add("refs/heads/next");
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refs.add("refs/tags/v1.0");
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Collections.sort(refs);
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SlowChecker slow = new SlowChecker(refs);
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FastChecker fast = new FastChecker(refs);
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String candidate = "refs/heads/feature";
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boolean slowResult = slow.collides(candidate);
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boolean fastResult = fast.collides(candidate);
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check("no-collision slow returns false", !slowResult);
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check("no-collision fast returns false", !fastResult);
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check("no-collision results agree", slowResult == fastResult);
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}
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// --- Correctness: collision (new ref is prefix of existing) ---
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{
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List<String> refs = new ArrayList<>();
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refs.add("refs/heads/foo/bar");
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refs.add("refs/heads/foo/baz");
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refs.add("refs/heads/zzz");
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Collections.sort(refs);
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SlowChecker slow = new SlowChecker(refs);
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FastChecker fast = new FastChecker(refs);
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// "refs/heads/foo" would be a directory component of existing refs
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String candidate = "refs/heads/foo";
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boolean slowResult = slow.collides(candidate);
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boolean fastResult = fast.collides(candidate);
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check("collision slow returns true", slowResult);
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check("collision fast returns true", fastResult);
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check("collision results agree", slowResult == fastResult);
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}
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// --- Correctness: near-miss (prefix but no slash) ---
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{
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List<String> refs = new ArrayList<>();
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refs.add("refs/heads/foobar");
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Collections.sort(refs);
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SlowChecker slow = new SlowChecker(refs);
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FastChecker fast = new FastChecker(refs);
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// "refs/heads/foo" is a STRING prefix of "refs/heads/foobar"
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// but NOT a directory prefix (no slash after "foo")
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String candidate = "refs/heads/foo";
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boolean slowResult = slow.collides(candidate);
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boolean fastResult = fast.collides(candidate);
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check("near-miss slow returns false", !slowResult);
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check("near-miss fast returns false", !fastResult);
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check("near-miss results agree", slowResult == fastResult);
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}
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// --- Correctness: empty cache ---
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{
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List<String> refs = new ArrayList<>();
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SlowChecker slow = new SlowChecker(refs);
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FastChecker fast = new FastChecker(refs);
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boolean slowResult = slow.collides("refs/heads/anything");
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boolean fastResult = fast.collides("refs/heads/anything");
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check("empty-cache slow returns false", !slowResult);
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check("empty-cache fast returns false", !fastResult);
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}
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// --- Correctness: candidate is before all entries ---
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{
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List<String> refs = new ArrayList<>();
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refs.add("refs/heads/zzz/child");
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Collections.sort(refs);
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SlowChecker slow = new SlowChecker(refs);
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FastChecker fast = new FastChecker(refs);
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boolean slowResult = slow.collides("refs/heads/aaa");
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boolean fastResult = fast.collides("refs/heads/aaa");
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check("before-all slow returns false", !slowResult);
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check("before-all fast returns false", !fastResult);
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check("before-all results agree", slowResult == fastResult);
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}
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// --- Performance: O(n²) vs O(n log n) ---
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{
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int R = 100_000;
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List<String> refs = buildPackedRefs(R);
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// Candidate that collides with last entry to force full scan in slow:
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// We pick a ref name that IS a prefix of many entries.
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// Add a colliding ref:
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refs.add("refs/remotes/origin/branch-0000000");
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// new_ref that would collide: "refs/remotes/origin" collides if
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// "refs/remotes/origin/..." exist — but "refs/remotes/origin" itself
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// is not in the list. Let's test a true no-collision near the end
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// to force full scan.
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Collections.sort(refs);
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// For slow, worst case: no collision but must scan all R entries.
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String noCollisionCandidate = "refs/zzz/new";
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SlowChecker slow = new SlowChecker(refs);
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FastChecker fast = new FastChecker(refs);
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long t0 = System.nanoTime();
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int slowRuns = 1000;
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for (int i = 0; i < slowRuns; i++) slow.collides(noCollisionCandidate);
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long slowNs = System.nanoTime() - t0;
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long t1 = System.nanoTime();
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int fastRuns = 1000;
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for (int i = 0; i < fastRuns; i++) fast.collides(noCollisionCandidate);
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long fastNs = System.nanoTime() - t1;
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// Slow must scan all R entries per call. Fast does O(log R).
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// We verify slow.lastOps = R, fast.ops = 1 (symbolic).
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boolean slowScansAll = (slow.lastOps == refs.size());
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boolean fastIsLogN = (fast.ops == 1);
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double ratio = (double) slowNs / fastNs;
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System.out.printf(" INFO slow=%d ops/call fast=O(logN) ratio=%.1fx%n",
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slow.lastOps, ratio);
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check("slow scans all R entries", slowScansAll);
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check("fast uses binary search", fastIsLogN);
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check("fast is meaningfully faster (>= 5x)", ratio >= 5.0);
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}
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System.out.println();
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System.out.printf("%d/%d PASS%n", passed, total);
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if (passed != total) System.exit(1);
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}
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}
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