fix(blacksmith-cache): disable auto-gc during fetch and clone to prevent unmount race (#23)

git fetch and clone are porcelain commands that trigger gc --auto internally
after completing. Since gc.autoDetach defaults to true, this spawns a background
daemon holding cwd and mmap'd pack files on the mirror mount, blocking the
subsequent umount with EBUSY even though our explicit runMirrorGC() uses
gc.autoDetach=false.

Disable auto-gc during fetch/clone with gc.auto=0 to eliminate the race. This
makes runMirrorGC() the single place gc runs, with proper timeout and error
handling. Fixes unmount failures in post-checkout cleanup (BLA-3202).

Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>

Author: adityamaru

dist/index.js

@@ -294,9 +294,12 @@ function ensureMirror(mirrorPath_1, repoUrl_1, authToken_1) {
                 core.info(`[git-mirror] Removing partial mirror directory from failed attempt`);
                 yield fs.promises.rm(mirrorPath, { recursive: true, force: true });
             }
+            // gc.auto=0: disable auto-gc during clone (see comment in refreshMirror)
             const cloneArgs = [
                 '-c',
                 `${configKey}=${configValue}`,
+                '-c',
+                'gc.auto=0',
                 'clone',
                 '--mirror',
                 '--progress',
@@ -333,9 +336,16 @@ function refreshMirror(mirrorPath_1, repoUrl_1, authToken_1) {
             const { configKey, configValue } = getAuthConfigArgs(repoUrl, authToken);
             const gitEnv = buildGitEnv(verbose);
             yield retryHelper.execute(() => __awaiter(this, void 0, void 0, function* () {
+                // gc.auto=0: disable git's internal auto-gc that porcelain commands like
+                // fetch run after completing. Without this, fetch can spawn a background
+                // gc daemon (gc.autoDetach defaults to true) that holds cwd + mmap'd pack
+                // files on the mirror mount, causing the subsequent umount to fail with
+                // EBUSY. We run gc explicitly in runMirrorGC() with gc.autoDetach=false.
                 const fetchArgs = [
                     '-c',
                     `${configKey}=${configValue}`,
+                    '-c',
+                    'gc.auto=0',
                     '-C',
                     mirrorPath,
                     'fetch',

src/blacksmith-cache.ts

@@ -322,9 +322,12 @@ export async function ensureMirror(
       )
       await fs.promises.rm(mirrorPath, {recursive: true, force: true})
     }
+    // gc.auto=0: disable auto-gc during clone (see comment in refreshMirror)
     const cloneArgs = [
       '-c',
       `${configKey}=${configValue}`,
+      '-c',
+      'gc.auto=0',
       'clone',
       '--mirror',
       '--progress',
@@ -372,9 +375,16 @@ export async function refreshMirror(
     const {configKey, configValue} = getAuthConfigArgs(repoUrl, authToken)
     const gitEnv = buildGitEnv(verbose)
     await retryHelper.execute(async () => {
+      // gc.auto=0: disable git's internal auto-gc that porcelain commands like
+      // fetch run after completing. Without this, fetch can spawn a background
+      // gc daemon (gc.autoDetach defaults to true) that holds cwd + mmap'd pack
+      // files on the mirror mount, causing the subsequent umount to fail with
+      // EBUSY. We run gc explicitly in runMirrorGC() with gc.autoDetach=false.
       const fetchArgs = [
         '-c',
         `${configKey}=${configValue}`,
+        '-c',
+        'gc.auto=0',
         '-C',
         mirrorPath,
         'fetch',