From be76e33474f7c87427ac25272194abdaa8ad8f7c Mon Sep 17 00:00:00 2001
From: Antoine Lassagne <antoine.lassagne@canonical.com>
Date: Thu, 22 Oct 2026 08:28:36 -0600
Subject: [PATCH] Drain tar stderr while streaming archives

Drain tar's stderr concurrently while writing archive data to its stdin.
Otherwise, diagnostics can fill the stderr pipe and deadlock both processes.
Drain stderr to EOF, retaining its first 4096 bytes for error reporting, and
join the drain thread before releasing the subprocess.
---
 src/as-utils.c | 59 +++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 49 insertions(+), 10 deletions(-)

diff --git a/src/as-utils.c b/src/as-utils.c
index a27860d8..e9b7fc7c 100644
--- a/src/as-utils.c
+++ b/src/as-utils.c
@@ -2493,6 +2493,27 @@ as_utils_tar_child_setup (gpointer user_data)
 }
 #endif
 
+/**
+ * as_utils_tar_stderr_drain_cb:
+ *
+ * Drain tar's stderr to EOF, retaining up to 4096 bytes for diagnostics.
+ */
+static gpointer
+as_utils_tar_stderr_drain_cb (gpointer data)
+{
+	GString *diagnostic = g_string_new ("");
+	gssize bytes_read;
+	gchar buffer[4096];
+
+	while ((bytes_read = g_input_stream_read (data, buffer, sizeof (buffer), NULL, NULL)) > 0) {
+		/* Keep draining even after the diagnostic buffer is full. */
+		g_string_append_len (diagnostic,
+				     buffer,
+				     MIN ((gsize) bytes_read, sizeof (buffer) - diagnostic->len));
+	}
+	return diagnostic;
+}
+
 /**
  * as_utils_extract_tarball:
  *
@@ -2508,6 +2529,9 @@ as_utils_extract_tarball (const gchar *filename, const gchar *target_dir, GError
 	g_autoptr(GSubprocessLauncher) launcher = NULL;
 	g_autoptr(GSubprocess) tar_process = NULL;
 	GOutputStream *tar_stdin = NULL;
+	g_autoptr(GString) stderr_buf = NULL;
+	GThread *stderr_thread;
+	gboolean ret = FALSE;
 	gssize bytes_read;
 	gchar buffer[4096];
 	GError *tmp_error = NULL;
@@ -2559,6 +2583,12 @@ as_utils_extract_tarball (const gchar *filename, const gchar *target_dir, GError
 		return FALSE;
 	}
 
+	/* Drain stderr concurrently so tar cannot block on a full pipe while
+	 * we write the archive to its stdin. */
+	stderr_thread = g_thread_new ("as-tar-stderr-drain",
+				      as_utils_tar_stderr_drain_cb,
+				      g_subprocess_get_stderr_pipe (tar_process));
+
 	tar_stdin = g_subprocess_get_stdin_pipe (tar_process);
 
 	/* read decompressed data from the tarball and write it to the tar subprocess */
@@ -2569,32 +2599,33 @@ as_utils_extract_tarball (const gchar *filename, const gchar *target_dir, GError
 						  &tmp_error)) > 0) {
 		if (!g_output_stream_write_all (tar_stdin, buffer, bytes_read, NULL, NULL, error)) {
 			g_prefix_error (error, "Error writing to tar subprocess: ");
-			return FALSE;
+			goto out;
 		}
 	}
 
 	/* close the input stream to tar */
 	if (!g_output_stream_close (tar_stdin, NULL, error)) {
 		g_prefix_error (error, "Error closing tar subprocess stdin: ");
-		return FALSE;
+		goto out;
 	}
 
 	if (tmp_error != NULL) {
 		g_propagate_prefixed_error (error, tmp_error, "Error reading tarball: ");
-		return FALSE;
+		goto out;
 	}
 
 	/* wait for the subprocess to finish and check for errors */
 	if (!g_subprocess_wait_check (tar_process, NULL, &tmp_error)) {
-		g_autoptr(GBytes) stderr_bytes = NULL;
 		g_autoptr(GString) error_msg = NULL;
 		const gchar *stderr_data;
 		gsize stderr_size;
-		GInputStream *tar_stderr = NULL;
 
-		tar_stderr = g_subprocess_get_stderr_pipe (tar_process);
-		stderr_bytes = g_input_stream_read_bytes (tar_stderr, 4096, NULL, NULL);
-		stderr_data = g_bytes_get_data (stderr_bytes, &stderr_size);
+		/* Process exit does not mean all buffered stderr has been consumed. */
+		stderr_buf = g_thread_join (stderr_thread);
+		stderr_thread = NULL;
+
+		stderr_data = stderr_buf->str;
+		stderr_size = stderr_buf->len;
 
 		error_msg = g_string_new ("Tarball extraction failed");
 		if (stderr_size > 0) {
@@ -2623,10 +2654,18 @@ as_utils_extract_tarball (const gchar *filename, const gchar *target_dir, GError
 				     g_subprocess_get_exit_status (tar_process));
 		}
 
-		return FALSE;
+		goto out;
 	}
 
-	return TRUE;
+	ret = TRUE;
+out:
+	/* Closing stdin lets tar exit before we join the drain thread. */
+	if (stderr_thread != NULL) {
+		if (!g_output_stream_is_closed (tar_stdin))
+			g_output_stream_close (tar_stdin, NULL, NULL);
+		stderr_buf = g_thread_join (stderr_thread);
+	}
+	return ret;
 }
 
 /**