Add built-in ERROR handling for archive callbacks.

Presently, the archiver process restarts when an archive callback
ERRORs.  To avoid this, archive module authors can use sigsetjmp(),
manage a memory context, etc., but that requires a lot of extra
code that will likely look roughly the same between modules.  This
commit adds basic archive callback ERROR handling to pgarch.c so
that module authors won't ordinarily need to worry about this.
While this built-in handler attempts to clean up anything that an
archive module could conceivably have left behind, it is possible
that some modules are doing unexpected things that require
additional cleanup.  Module authors should be sure to do any extra
required cleanup in a PG_CATCH block within the archiving callback.

The archiving callback is now called in a short-lived memory
context that the archiver process resets between invocations.  If a
module requires longer-lived storage, it must maintain its own
memory context.

Thanks to these changes, the basic_archive module can be greatly
simplified.

Suggested-by: Andres Freund
Reviewed-by: Andres Freund, Yong Li
Discussion: https://postgr.es/m/20230217215624.GA3131134%40nathanxps13

Author: nathan-bossart

contrib/basic_archive/basic_archive.c

@@ -40,26 +40,18 @@
 
 PG_MODULE_MAGIC;
 
-typedef struct BasicArchiveData
-{
-	MemoryContext context;
-} BasicArchiveData;
-
 static char *archive_directory = NULL;
 
-static void basic_archive_startup(ArchiveModuleState *state);
 static bool basic_archive_configured(ArchiveModuleState *state);
 static bool basic_archive_file(ArchiveModuleState *state, const char *file, const char *path);
-static void basic_archive_file_internal(const char *file, const char *path);
 static bool check_archive_directory(char **newval, void **extra, GucSource source);
 static bool compare_files(const char *file1, const char *file2);
-static void basic_archive_shutdown(ArchiveModuleState *state);
 
 static const ArchiveModuleCallbacks basic_archive_callbacks = {
-	.startup_cb = basic_archive_startup,
+	.startup_cb = NULL,
 	.check_configured_cb = basic_archive_configured,
 	.archive_file_cb = basic_archive_file,
-	.shutdown_cb = basic_archive_shutdown
+	.shutdown_cb = NULL
 };
 
 /*
@@ -93,24 +85,6 @@ _PG_archive_module_init(void)
 	return &basic_archive_callbacks;
 }
 
-/*
- * basic_archive_startup
- *
- * Creates the module's memory context.
- */
-void
-basic_archive_startup(ArchiveModuleState *state)
-{
-	BasicArchiveData *data;
-
-	data = (BasicArchiveData *) MemoryContextAllocZero(TopMemoryContext,
-													   sizeof(BasicArchiveData));
-	data->context = AllocSetContextCreate(TopMemoryContext,
-										  "basic_archive",
-										  ALLOCSET_DEFAULT_SIZES);
-	state->private_data = (void *) data;
-}
-
 /*
  * check_archive_directory
  *
@@ -176,74 +150,6 @@ basic_archive_configured(ArchiveModuleState *state)
  */
 static bool
 basic_archive_file(ArchiveModuleState *state, const char *file, const char *path)
-{
-	sigjmp_buf	local_sigjmp_buf;
-	MemoryContext oldcontext;
-	BasicArchiveData *data = (BasicArchiveData *) state->private_data;
-	MemoryContext basic_archive_context = data->context;
-
-	/*
-	 * We run basic_archive_file_internal() in our own memory context so that
-	 * we can easily reset it during error recovery (thus avoiding memory
-	 * leaks).
-	 */
-	oldcontext = MemoryContextSwitchTo(basic_archive_context);
-
-	/*
-	 * Since the archiver operates at the bottom of the exception stack,
-	 * ERRORs turn into FATALs and cause the archiver process to restart.
-	 * However, using ereport(ERROR, ...) when there are problems is easy to
-	 * code and maintain.  Therefore, we create our own exception handler to
-	 * catch ERRORs and return false instead of restarting the archiver
-	 * whenever there is a failure.
-	 */
-	if (sigsetjmp(local_sigjmp_buf, 1) != 0)
-	{
-		/* Since not using PG_TRY, must reset error stack by hand */
-		error_context_stack = NULL;
-
-		/* Prevent interrupts while cleaning up */
-		HOLD_INTERRUPTS();
-
-		/* Report the error and clear ErrorContext for next time */
-		EmitErrorReport();
-		FlushErrorState();
-
-		/* Close any files left open by copy_file() or compare_files() */
-		AtEOSubXact_Files(false, InvalidSubTransactionId, InvalidSubTransactionId);
-
-		/* Reset our memory context and switch back to the original one */
-		MemoryContextSwitchTo(oldcontext);
-		MemoryContextReset(basic_archive_context);
-
-		/* Remove our exception handler */
-		PG_exception_stack = NULL;
-
-		/* Now we can allow interrupts again */
-		RESUME_INTERRUPTS();
-
-		/* Report failure so that the archiver retries this file */
-		return false;
-	}
-
-	/* Enable our exception handler */
-	PG_exception_stack = &local_sigjmp_buf;
-
-	/* Archive the file! */
-	basic_archive_file_internal(file, path);
-
-	/* Remove our exception handler */
-	PG_exception_stack = NULL;
-
-	/* Reset our memory context and switch back to the original one */
-	MemoryContextSwitchTo(oldcontext);
-	MemoryContextReset(basic_archive_context);
-
-	return true;
-}
-
-static void
-basic_archive_file_internal(const char *file, const char *path)
 {
 	char		destination[MAXPGPATH];
 	char		temp[MAXPGPATH + 256];
@@ -277,7 +183,7 @@ basic_archive_file_internal(const char *file, const char *path)
 			fsync_fname(destination, false);
 			fsync_fname(archive_directory, true);
 
-			return;
+			return true;
 		}
 
 		ereport(ERROR,
@@ -317,6 +223,8 @@ basic_archive_file_internal(const char *file, const char *path)
 
 	ereport(DEBUG1,
 			(errmsg("archived \"%s\" via basic_archive", file)));
+
+	return true;
 }
 
 /*
@@ -399,35 +307,3 @@ compare_files(const char *file1, const char *file2)
 
 	return ret;
 }
-
-/*
- * basic_archive_shutdown
- *
- * Frees our allocated state.
- */
-static void
-basic_archive_shutdown(ArchiveModuleState *state)
-{
-	BasicArchiveData *data = (BasicArchiveData *) state->private_data;
-	MemoryContext basic_archive_context;
-
-	/*
-	 * If we didn't get to storing the pointer to our allocated state, we
-	 * don't have anything to clean up.
-	 */
-	if (data == NULL)
-		return;
-
-	basic_archive_context = data->context;
-	Assert(CurrentMemoryContext != basic_archive_context);
-
-	if (MemoryContextIsValid(basic_archive_context))
-		MemoryContextDelete(basic_archive_context);
-	data->context = NULL;
-
-	/*
-	 * Finally, free the state.
-	 */
-	pfree(data);
-	state->private_data = NULL;
-}

doc/src/sgml/archive-modules.sgml

@@ -140,12 +140,21 @@ typedef bool (*ArchiveFileCB) (ArchiveModuleState *state, const char *file, cons
 
     If <literal>true</literal> is returned, the server proceeds as if the file
     was successfully archived, which may include recycling or removing the
-    original WAL file.  If <literal>false</literal> is returned, the server will
+    original WAL file.  If <literal>false</literal> is returned or an error is thrown, the server will
     keep the original WAL file and retry archiving later.
     <replaceable>file</replaceable> will contain just the file name of the WAL
     file to archive, while <replaceable>path</replaceable> contains the full
     path of the WAL file (including the file name).
    </para>
+
+   <note>
+    <para>
+     The <function>archive_file_cb</function> callback is called in a
+     short-lived memory context that will be reset between invocations.  If you
+     need longer-lived storage, create a memory context in the module's
+     <function>startup_cb</function> callback.
+    </para>
+   </note>
   </sect2>
 
   <sect2 id="archive-module-shutdown">

src/backend/archive/shell_archive.c

@@ -71,9 +71,6 @@ shell_archive_file(ArchiveModuleState *state, const char *file,
 											   "archive_command", "fp",
 											   file, nativePath);
 
-	if (nativePath)
-		pfree(nativePath);
-
 	ereport(DEBUG3,
 			(errmsg_internal("executing archive command \"%s\"",
 							 xlogarchcmd)));
@@ -132,8 +129,6 @@ shell_archive_file(ArchiveModuleState *state, const char *file,
 		return false;
 	}
 
-	pfree(xlogarchcmd);
-
 	elog(DEBUG1, "archived write-ahead log file \"%s\"", file);
 	return true;
 }

src/backend/postmaster/pgarch.c

@@ -39,6 +39,7 @@
 #include "postmaster/auxprocess.h"
 #include "postmaster/interrupt.h"
 #include "postmaster/pgarch.h"
+#include "storage/condition_variable.h"
 #include "storage/fd.h"
 #include "storage/ipc.h"
 #include "storage/latch.h"
@@ -49,6 +50,8 @@
 #include "utils/guc.h"
 #include "utils/memutils.h"
 #include "utils/ps_status.h"
+#include "utils/resowner.h"
+#include "utils/timeout.h"
 
 
 /* ----------
@@ -100,6 +103,7 @@ static time_t last_sigterm_time = 0;
 static PgArchData *PgArch = NULL;
 static const ArchiveModuleCallbacks *ArchiveCallbacks;
 static ArchiveModuleState *archive_module_state;
+static MemoryContext archive_context;
 
 
 /*
@@ -257,6 +261,11 @@ PgArchiverMain(char *startup_data, size_t startup_data_len)
 												ready_file_comparator, false,
 												NULL);
 
+	/* Initialize our memory context. */
+	archive_context = AllocSetContextCreate(TopMemoryContext,
+											"archiver",
+											ALLOCSET_DEFAULT_SIZES);
+
 	/* Load the archive_library. */
 	LoadArchiveLibrary();
 
@@ -507,6 +516,8 @@ pgarch_ArchiverCopyLoop(void)
 static bool
 pgarch_archiveXlog(char *xlog)
 {
+	sigjmp_buf	local_sigjmp_buf;
+	MemoryContext oldcontext;
 	char		pathname[MAXPGPATH];
 	char		activitymsg[MAXFNAMELEN + 16];
 	bool		ret;
@@ -517,7 +528,87 @@ pgarch_archiveXlog(char *xlog)
 	snprintf(activitymsg, sizeof(activitymsg), "archiving %s", xlog);
 	set_ps_display(activitymsg);
 
-	ret = ArchiveCallbacks->archive_file_cb(archive_module_state, xlog, pathname);
+	oldcontext = MemoryContextSwitchTo(archive_context);
+
+	/*
+	 * Since the archiver operates at the bottom of the exception stack,
+	 * ERRORs turn into FATALs and cause the archiver process to restart.
+	 * However, using ereport(ERROR, ...) when there are problems is easy to
+	 * code and maintain.  Therefore, we create our own exception handler to
+	 * catch ERRORs and return false instead of restarting the archiver
+	 * whenever there is a failure.
+	 *
+	 * We assume ERRORs from the archiving callback are the most common
+	 * exceptions experienced by the archiver, so we opt to handle exceptions
+	 * here instead of PgArchiverMain() to avoid reinitializing the archiver
+	 * too frequently.  We could instead add a sigsetjmp() block to
+	 * PgArchiverMain() and use PG_TRY/PG_CATCH here, but the extra code to
+	 * avoid the odd archiver restart doesn't seem worth it.
+	 */
+	if (sigsetjmp(local_sigjmp_buf, 1) != 0)
+	{
+		/* Since not using PG_TRY, must reset error stack by hand */
+		error_context_stack = NULL;
+
+		/* Prevent interrupts while cleaning up */
+		HOLD_INTERRUPTS();
+
+		/* Report the error to the server log. */
+		EmitErrorReport();
+
+		/*
+		 * Try to clean up anything the archive module left behind.  We try to
+		 * cover anything that an archive module could conceivably have left
+		 * behind, but it is of course possible that modules could be doing
+		 * unexpected things that require additional cleanup.  Module authors
+		 * should be sure to do any extra required cleanup in a PG_CATCH block
+		 * within the archiving callback, and they are encouraged to notify
+		 * the pgsql-hackers mailing list so that we can add it here.
+		 */
+		disable_all_timeouts(false);
+		LWLockReleaseAll();
+		ConditionVariableCancelSleep();
+		pgstat_report_wait_end();
+		ReleaseAuxProcessResources(false);
+		AtEOXact_Files(false);
+		AtEOXact_HashTables(false);
+
+		/*
+		 * Return to the original memory context and clear ErrorContext for
+		 * next time.
+		 */
+		MemoryContextSwitchTo(oldcontext);
+		FlushErrorState();
+
+		/* Flush any leaked data */
+		MemoryContextReset(archive_context);
+
+		/* Remove our exception handler */
+		PG_exception_stack = NULL;
+
+		/* Now we can allow interrupts again */
+		RESUME_INTERRUPTS();
+
+		/* Report failure so that the archiver retries this file */
+		ret = false;
+	}
+	else
+	{
+		/* Enable our exception handler */
+		PG_exception_stack = &local_sigjmp_buf;
+
+		/* Archive the file! */
+		ret = ArchiveCallbacks->archive_file_cb(archive_module_state,
+												xlog, pathname);
+
+		/* Remove our exception handler */
+		PG_exception_stack = NULL;
+
+		/* Reset our memory context and switch back to the original one */
+		MemoryContextSwitchTo(oldcontext);
+		MemoryContextReset(archive_context);
+	}
+
 	if (ret)
 		snprintf(activitymsg, sizeof(activitymsg), "last was %s", xlog);
 	else