fix(blacksmith-cache): prevent git gc background daemon from blocking unmount (#22)

git gc --auto defaults to gc.autoDetach=true, which forks a background daemon
for the actual garbage collection work. The parent returns immediately with
exit code 0, but the daemonized child (and its sub-processes) keep running with
cwd and mmap'd pack files on the mirror mount. This causes subsequent umount
attempts to fail with EBUSY (exit code 32).

Root cause identified in descriptinc/descript: 4 git processes (PIDs 10316-10319)
with cwd inside the mirror, preventing unmount despite 3 retry attempts with
exponential backoff.

Fix: Pass -c gc.autoDetach=false to force synchronous GC execution so it fully
completes before proceeding to mount cleanup.

Also increase GC_TIMEOUT_SECS from 60s to 120s: measured GC on this repo takes
~74 seconds to repack 51 packs + 1019 loose objects.

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

Author: adityamaru

dist/index.js

@@ -60,7 +60,7 @@ const GRPC_PORT = process.env.BLACKSMITH_STICKY_DISK_GRPC_PORT || '5557';
 const MOUNT_BASE = '/blacksmith-git-mirror';
 const MIRROR_VERSION = 'v1';
 const REFRESH_TIMEOUT_SECS = 120; // 2 minutes
-const GC_TIMEOUT_SECS = 60; // 60 seconds
+const GC_TIMEOUT_SECS = 120; // 2 minutes
 const FLUSH_TIMEOUT_SECS = 10; // 10 seconds for durability flush
 const UMOUNT_TIMEOUT_SECS = 10; // 10 seconds for unmount
 const UMOUNT_MAX_RETRIES = 3; // Number of unmount retry attempts
@@ -417,7 +417,20 @@ function runMirrorGC(mirrorPath_1) {
         try {
             // --auto: only run if thresholds exceeded (default: 6700 loose objects or 50 packs)
             // This is much faster than a full gc when not needed
-            const result = yield exec.getExecOutput('timeout', [String(timeoutSecs), 'git', '-C', mirrorPath, 'gc', '--auto'], { ignoreReturnCode: true });
+            // gc.autoDetach=false: prevent git from forking a background daemon for GC.
+            // Without this, the parent `git gc --auto` returns immediately while the
+            // daemonized child keeps running with cwd and mmap'd pack files on the
+            // mirror mount, causing the subsequent `umount` to fail with EBUSY.
+            const result = yield exec.getExecOutput('timeout', [
+                String(timeoutSecs),
+                'git',
+                '-c',
+                'gc.autoDetach=false',
+                '-C',
+                mirrorPath,
+                'gc',
+                '--auto'
+            ], { ignoreReturnCode: true });
             if (result.exitCode === TIMEOUT_EXIT_CODE) {
                 core.warning(`[git-mirror] GC timed out after ${timeoutSecs}s`);
                 return {

src/blacksmith-cache.ts

@@ -12,7 +12,7 @@ const MOUNT_BASE = '/blacksmith-git-mirror'
 const MIRROR_VERSION = 'v1'
 
 const REFRESH_TIMEOUT_SECS = 120 // 2 minutes
-const GC_TIMEOUT_SECS = 60 // 60 seconds
+const GC_TIMEOUT_SECS = 120 // 2 minutes
 const FLUSH_TIMEOUT_SECS = 10 // 10 seconds for durability flush
 const UMOUNT_TIMEOUT_SECS = 10 // 10 seconds for unmount
 const UMOUNT_MAX_RETRIES = 3 // Number of unmount retry attempts
@@ -479,9 +479,22 @@ async function runMirrorGC(
   try {
     // --auto: only run if thresholds exceeded (default: 6700 loose objects or 50 packs)
     // This is much faster than a full gc when not needed
+    // gc.autoDetach=false: prevent git from forking a background daemon for GC.
+    // Without this, the parent `git gc --auto` returns immediately while the
+    // daemonized child keeps running with cwd and mmap'd pack files on the
+    // mirror mount, causing the subsequent `umount` to fail with EBUSY.
     const result = await exec.getExecOutput(
       'timeout',
-      [String(timeoutSecs), 'git', '-C', mirrorPath, 'gc', '--auto'],
+      [
+        String(timeoutSecs),
+        'git',
+        '-c',
+        'gc.autoDetach=false',
+        '-C',
+        mirrorPath,
+        'gc',
+        '--auto'
+      ],
       {ignoreReturnCode: true}
     )
     if (result.exitCode === TIMEOUT_EXIT_CODE) {