Recover from crash using undo log

This commit remembers the file write position of a last successful transaction in
a undo file. If there's a crash and we need to restart, we use this undo file to get rid of any partial work from the last transaction that wasn't finished.

TODO: Can we use the undo log mechanism to even for graceful exit?
This would simplify the code and make it uniform for both graceful
and ungraceful exits/crashes. The idea is to undo the changes made
by the last incomplete transaction by reading the undo log file and
write other messages like ENDPOS.

Signed-off-by: Arunprasad Rajkumar <[email protected]>

src/bin/pgcopydb/copydb.c

@@ -572,6 +572,10 @@ copydb_prepare_filepaths(CopyFilePaths *cfPaths,
 			"%s/lsn.json",
 			cfPaths->cdc.dir);
 
+	sformat(cfPaths->cdc.undofile, MAXPGPATH,
+			"%s/undo",
+			cfPaths->cdc.dir);
+
 	/*
 	 * Now prepare the "compare" files we need to compare schema and data
 	 * between the source and target instance.

src/bin/pgcopydb/copydb.h

@@ -26,6 +26,7 @@
 	{ "extra_float_digits", "3" }, \
 	{ "statement_timeout", "0" }, \
 	{ "default_transaction_read_only", "off" }
+
 /*
  * These parameters are added to the connection strings, unless the user has
  * added them, allowing user-defined values to be taken into account.

src/bin/pgcopydb/copydb_paths.h

@@ -54,6 +54,7 @@ typedef struct CDCPaths
 	char tlifile[MAXPGPATH];          /* /tmp/pgcopydb/cdc/tli */
 	char tlihistfile[MAXPGPATH];      /* /tmp/pgcopydb/cdc/tli.history */
 	char lsntrackingfile[MAXPGPATH];  /* /tmp/pgcopydb/cdc/lsn.json */
+	char undofile[MAXPGPATH];         /* /tmp/pgcopydb/cdc/undo */
 } CDCPaths;
 
 

src/bin/pgcopydb/follow.c

@@ -257,6 +257,22 @@ follow_get_sentinel(StreamSpecs *specs, CopyDBSentinel *sentinel, bool verbose)
 bool
 follow_main_loop(CopyDataSpec *copySpecs, StreamSpecs *streamSpecs)
 {
+	/*
+	 * Incase of a crash, execute recovery actions before starting the
+	 * main loop.
+	 */
+	if (!recoverFromUndoLog(&streamSpecs->paths))
+	{
+		log_error("Failed to recover from undo log, see above for details");
+		return false;
+	}
+
+	if (!removeUndoLog(&streamSpecs->paths))
+	{
+		log_error("Failed to remove undo log, see above for details");
+		return false;
+	}
+
 	/*
 	 * Remove the possibly still existing stream context files from
 	 * previous round of operations (--resume, etc). We want to make