Fix foreground commands blocking on background processes

.changeset/neat-drinks-own.md

@@ -0,0 +1,5 @@
+---
+"@cloudflare/sandbox": patch
+---
+
+Fix foreground commands blocking on background processes

docs/SESSION_EXECUTION.md

@@ -14,22 +14,20 @@ This document explains how the container session executes commands reliably whil
 ### Foreground (`exec`)
 
 - Runs in the main bash shell so state persists across commands.
-- Uses bash process substitution to prefix stdout/stderr inline and append to the per-command log file.
-- After the command returns, we `wait` to ensure process-substitution consumers finish writing to the log before we write the exit code file.
-- Why process substitution (not FIFOs):
-  - Foreground previously used FIFOs + background labelers, which can race on silent commands because FIFO open/close ordering depends on cross-process scheduling.
-  - Process substitution keeps execution local to the main shell and avoids FIFO semantics entirely.
+- Writes stdout/stderr to temporary files, then prefixes and merges them into the log.
+- Bash waits for file redirects to complete before continuing, ensuring the log is fully written before the exit code is published.
+- This avoids race conditions from process substitution buffering where log reads could happen before writes complete.
 
 Pseudo:
 
 ```
 # Foreground
-{ command; } \
-  > >(while read; printf "\x01\x01\x01%s\n" "$REPLY" >> "$log") \
-  2> >(while read; printf "\x02\x02\x02%s\n" "$REPLY" >> "$log")
+{ command; } > "$log.stdout" 2> "$log.stderr"
 EXIT_CODE=$?
-# Ensure consumers have drained
-wait 2>/dev/null
+# Prefix and merge into main log
+(while read line; do printf "\x01\x01\x01%s\n" "$line"; done < "$log.stdout") >> "$log"
+(while read line; do printf "\x02\x02\x02%s\n" "$line"; done < "$log.stderr") >> "$log"
+rm -f "$log.stdout" "$log.stderr"
 # Atomically publish exit code
 echo "$EXIT_CODE" > "$exit.tmp" && mv "$exit.tmp" "$exit"
 ```
@@ -97,3 +95,5 @@ mkfifo "$sp" "$ep"
 - Why not tee? Tee doesn’t split stdout/stderr into separate channels with stable ordering without extra plumbing; our prefixes are simple and explicit.
 - Is process substitution portable?
   - It is supported by bash (we spawn bash with `--norc`). The container environment supports it; if portability constraints change, we can revisit.
+- Why use temp files instead of process substitution for foreground?
+  Process substitutions run asynchronously - bash returns when the substitution processes close, but their writes to the log file may still be buffered. With large output (e.g., base64-encoded files), the log file can be incomplete when we try to read it. Using direct file redirects ensures bash waits for all writes to complete before continuing, eliminating this race condition.

packages/sandbox-container/src/session.ts

@@ -722,17 +722,17 @@ export class Session {
       // FOREGROUND PATTERN (for exec)
       // Command runs in main shell, state persists!
 
-      // FOREGROUND: Avoid FIFOs to eliminate race conditions.
-      // Use bash process substitution to prefix stdout/stderr while keeping
-      // execution in the main shell so session state persists across commands.
+      // FOREGROUND: Write stdout/stderr to temp files, then prefix and merge.
+      // This ensures bash waits for all writes to complete before continuing,
+      // avoiding race conditions when reading the log file.
 
       if (cwd) {
         const safeCwd = this.escapeShellPath(cwd);
         script += `  # Save and change directory\n`;
         script += `  PREV_DIR=$(pwd)\n`;
         script += `  if cd ${safeCwd}; then\n`;
-        script += `    # Execute command with prefixed streaming via process substitution\n`;
-        script += `    { ${command}; } < /dev/null > >(while IFS= read -r line || [[ -n "$line" ]]; do printf '\\x01\\x01\\x01%s\\n' "$line"; done >> "$log") 2> >(while IFS= read -r line || [[ -n "$line" ]]; do printf '\\x02\\x02\\x02%s\\n' "$line"; done >> "$log")\n`;
+        script += `    # Execute command, redirect to temp files\n`;
+        script += `    { ${command}; } < /dev/null > "$log.stdout" 2> "$log.stderr"\n`;
         script += `    EXIT_CODE=$?\n`;
         script += `    # Restore directory\n`;
         script += `    cd "$PREV_DIR"\n`;
@@ -741,13 +741,16 @@ export class Session {
         script += `    EXIT_CODE=1\n`;
         script += `  fi\n`;
       } else {
-        script += `  # Execute command with prefixed streaming via process substitution\n`;
-        script += `  { ${command}; } < /dev/null > >(while IFS= read -r line || [[ -n "$line" ]]; do printf '\\x01\\x01\\x01%s\\n' "$line"; done >> "$log") 2> >(while IFS= read -r line || [[ -n "$line" ]]; do printf '\\x02\\x02\\x02%s\\n' "$line"; done >> "$log")\n`;
+        script += `  # Execute command, redirect to temp files\n`;
+        script += `  { ${command}; } < /dev/null > "$log.stdout" 2> "$log.stderr"\n`;
         script += `  EXIT_CODE=$?\n`;
       }
 
-      // Ensure process-substitution consumers complete before writing exit code
-      script += `  wait 2>/dev/null\n`;
+      script += `  \n`;
+      script += `  # Prefix and merge stdout/stderr into main log\n`;
+      script += `  (while IFS= read -r line || [[ -n "$line" ]]; do printf '\\x01\\x01\\x01%s\\n' "$line"; done < "$log.stdout" >> "$log") 2>/dev/null\n`;
+      script += `  (while IFS= read -r line || [[ -n "$line" ]]; do printf '\\x02\\x02\\x02%s\\n' "$line"; done < "$log.stderr" >> "$log") 2>/dev/null\n`;
+      script += `  rm -f "$log.stdout" "$log.stderr"\n`;
       script += `  \n`;
       script += `  # Write exit code\n`;
       script += `  echo "$EXIT_CODE" > ${safeExitCodeFile}.tmp\n`;

tests/e2e/process-lifecycle-workflow.test.ts

@@ -186,6 +186,61 @@ describe('Process Lifecycle Workflow', () => {
       });
     }, 90000);
 
+    test('should not block foreground operations when background processes are running', async () => {
+      const sandboxId = createSandboxId();
+      const headers = createTestHeaders(sandboxId);
+
+      // Start a long-running background process
+      const startResponse = await vi.waitFor(
+        async () =>
+          fetchWithStartup(`${workerUrl}/api/process/start`, {
+            method: 'POST',
+            headers,
+            body: JSON.stringify({
+              command: 'sleep 60'
+            })
+          }),
+        { timeout: 90000, interval: 2000 }
+      );
+
+      const startData = await startResponse.json();
+      const processId = startData.id;
+
+      // Immediately run a foreground command - should complete quickly
+      const execStart = Date.now();
+      const execResponse = await fetch(`${workerUrl}/api/execute`, {
+        method: 'POST',
+        headers,
+        body: JSON.stringify({
+          command: 'echo "test"'
+        })
+      });
+      const execDuration = Date.now() - execStart;
+
+      expect(execResponse.status).toBe(200);
+      expect(execDuration).toBeLessThan(2000); // Should complete in <2s, not wait for sleep
+
+      // Test listFiles as well - it uses the same foreground execution path
+      const listStart = Date.now();
+      const listResponse = await fetch(`${workerUrl}/api/list-files`, {
+        method: 'POST',
+        headers,
+        body: JSON.stringify({
+          path: '/workspace'
+        })
+      });
+      const listDuration = Date.now() - listStart;
+
+      expect(listResponse.status).toBe(200);
+      expect(listDuration).toBeLessThan(2000); // Should complete quickly
+
+      // Cleanup
+      await fetch(`${workerUrl}/api/process/${processId}`, {
+        method: 'DELETE',
+        headers
+      });
+    }, 90000);
+
     test('should get process logs after execution', async () => {
       const sandboxId = createSandboxId();
       const headers = createTestHeaders(sandboxId);