Fix some issues with WAL segment opening for pg_receivewal --compress

The logic handling the opening of new WAL segments was fuzzy when using
--compress if a partial, non-compressed, segment with the same base name
existed in the repository storing those files.  In this case, using
--compress would cause the code to first check for the existence and the
size of a non-compressed segment, followed by the opening of a new
compressed, partial, segment.  The code was accidentally working
correctly on most platforms as the buildfarm has proved, except
bowerbird where gzflush() could fail in this code path.  It is wrong
anyway to take the code path used pre-padding when creating a new
partial, non-compressed, segment, so let's fix it.

Note that this issue exists when users mix successive runs of
pg_receivewal with or without compression, as discovered with the tests
introduced by ffc9dda.

While on it, this refactors the code so as code paths that need to know
about the ".gz" suffix are down from four to one in walmethods.c, easing
a bit the introduction of new compression methods.  This addresses a
second issue where log messages generated for an unexpected failure
would not show the compressed segment name involved, which was
confusing, printing instead the name of the non-compressed equivalent.

Reported-by: Georgios Kokolatos
Discussion: https://postgr.es/m/YPDLz2x3o1aX2wRh@paquier.xyz
Backpatch-through: 10

src/bin/pg_basebackup/receivelog.c

@@ -88,26 +88,29 @@ static bool
 open_walfile(StreamCtl *stream, XLogRecPtr startpoint)
 {
 	Walfile    *f;
-	char		fn[MAXPGPATH];
+	char	   *fn;
 	ssize_t		size;
 	XLogSegNo	segno;
 
 	XLByteToSeg(startpoint, segno, WalSegSz);
 	XLogFileName(current_walfile_name, stream->timeline, segno, WalSegSz);
 
-	snprintf(fn, sizeof(fn), "%s%s", current_walfile_name,
-			 stream->partial_suffix ? stream->partial_suffix : "");
+	/* Note that this considers the compression used if necessary */
+	fn = stream->walmethod->get_file_name(current_walfile_name,
+										  stream->partial_suffix);
 
 	/*
 	 * When streaming to files, if an existing file exists we verify that it's
 	 * either empty (just created), or a complete WalSegSz segment (in which
 	 * case it has been created and padded). Anything else indicates a corrupt
-	 * file.
+	 * file. Compressed files have no need for padding, so just ignore this
+	 * case.
 	 *
 	 * When streaming to tar, no file with this name will exist before, so we
 	 * never have to verify a size.
 	 */
-	if (stream->walmethod->existsfile(fn))
+	if (stream->walmethod->compression() == 0 &&
+		stream->walmethod->existsfile(fn))
 	{
 		size = stream->walmethod->get_file_size(fn);
 		if (size < 0)

src/bin/pg_basebackup/walmethods.c

@@ -68,20 +68,32 @@ dir_getlasterror(void)
 	return strerror(errno);
 }
 
+static char *
+dir_get_file_name(const char *pathname, const char *temp_suffix)
+{
+	char	   *filename = pg_malloc0(MAXPGPATH * sizeof(char));
+
+	snprintf(filename, MAXPGPATH, "%s%s%s",
+			 pathname, dir_data->compression > 0 ? ".gz" : "",
+			 temp_suffix ? temp_suffix : "");
+
+	return filename;
+}
+
 static Walfile
 dir_open_for_write(const char *pathname, const char *temp_suffix, size_t pad_to_size)
 {
 	static char tmppath[MAXPGPATH];
+	char	   *filename;
 	int			fd;
 	DirectoryMethodFile *f;
 #ifdef HAVE_LIBZ
 	gzFile		gzfp = NULL;
 #endif
 
-	snprintf(tmppath, sizeof(tmppath), "%s/%s%s%s",
-			 dir_data->basedir, pathname,
-			 dir_data->compression > 0 ? ".gz" : "",
-			 temp_suffix ? temp_suffix : "");
+	filename = dir_get_file_name(pathname, temp_suffix);
+	snprintf(tmppath, sizeof(tmppath), "%s/%s",
+			 dir_data->basedir, filename);
 
 	/*
 	 * Open a file for non-compressed as well as compressed files. Tracking
@@ -232,26 +244,31 @@ dir_close(Walfile f, WalCloseMethod method)
 		/* Build path to the current version of the file */
 		if (method == CLOSE_NORMAL && df->temp_suffix)
 		{
+			char	   *filename;
+			char	   *filename2;
+
 			/*
 			 * If we have a temp prefix, normal operation is to rename the
 			 * file.
 			 */
-			snprintf(tmppath, sizeof(tmppath), "%s/%s%s%s",
-					 dir_data->basedir, df->pathname,
-					 dir_data->compression > 0 ? ".gz" : "",
-					 df->temp_suffix);
-			snprintf(tmppath2, sizeof(tmppath2), "%s/%s%s",
-					 dir_data->basedir, df->pathname,
-					 dir_data->compression > 0 ? ".gz" : "");
+			filename = dir_get_file_name(df->pathname, df->temp_suffix);
+			snprintf(tmppath, sizeof(tmppath), "%s/%s",
+					 dir_data->basedir, filename);
+
+			/* permanent name, so no need for the prefix */
+			filename2 = dir_get_file_name(df->pathname, NULL);
+			snprintf(tmppath2, sizeof(tmppath2), "%s/%s",
+					 dir_data->basedir, filename2);
 			r = durable_rename(tmppath, tmppath2);
 		}
 		else if (method == CLOSE_UNLINK)
 		{
+			char	   *filename;
+
 			/* Unlink the file once it's closed */
-			snprintf(tmppath, sizeof(tmppath), "%s/%s%s%s",
-					 dir_data->basedir, df->pathname,
-					 dir_data->compression > 0 ? ".gz" : "",
-					 df->temp_suffix ? df->temp_suffix : "");
+			filename = dir_get_file_name(df->pathname, df->temp_suffix);
+			snprintf(tmppath, sizeof(tmppath), "%s/%s",
+					 dir_data->basedir, filename);
 			r = unlink(tmppath);
 		}
 		else
@@ -313,6 +330,12 @@ dir_get_file_size(const char *pathname)
 	return statbuf.st_size;
 }
 
+static int
+dir_compression(void)
+{
+	return dir_data->compression;
+}
+
 static bool
 dir_existsfile(const char *pathname)
 {
@@ -355,6 +378,8 @@ CreateWalDirectoryMethod(const char *basedir, int compression, bool sync)
 	method->write = dir_write;
 	method->get_current_pos = dir_get_current_pos;
 	method->get_file_size = dir_get_file_size;
+	method->get_file_name = dir_get_file_name;
+	method->compression = dir_compression;
 	method->close = dir_close;
 	method->sync = dir_sync;
 	method->existsfile = dir_existsfile;
@@ -527,11 +552,22 @@ tar_write_padding_data(TarMethodFile *f, size_t bytes)
 	return true;
 }
 
+static char *
+tar_get_file_name(const char *pathname, const char *temp_suffix)
+{
+	char	   *filename = pg_malloc0(MAXPGPATH * sizeof(char));
+
+	snprintf(filename, MAXPGPATH, "%s%s",
+			 pathname, temp_suffix ? temp_suffix : "");
+
+	return filename;
+}
+
 static Walfile
 tar_open_for_write(const char *pathname, const char *temp_suffix, size_t pad_to_size)
 {
 	int			save_errno;
-	static char tmppath[MAXPGPATH];
+	char	   *tmppath;
 
 	tar_clear_error();
 
@@ -583,8 +619,7 @@ tar_open_for_write(const char *pathname, const char *temp_suffix, size_t pad_to_
 
 	tar_data->currentfile = pg_malloc0(sizeof(TarMethodFile));
 
-	snprintf(tmppath, sizeof(tmppath), "%s%s",
-			 pathname, temp_suffix ? temp_suffix : "");
+	tmppath = tar_get_file_name(pathname, temp_suffix);
 
 	/* Create a header with size set to 0 - we will fill out the size on close */
 	if (tarCreateHeader(tar_data->currentfile->header, tmppath, NULL, 0, S_IRUSR | S_IWUSR, 0, 0, time(NULL)) != TAR_OK)
@@ -689,6 +724,12 @@ tar_get_file_size(const char *pathname)
 	return -1;
 }
 
+static int
+tar_compression(void)
+{
+	return tar_data->compression;
+}
+
 static off_t
 tar_get_current_pos(Walfile f)
 {
@@ -994,6 +1035,8 @@ CreateWalTarMethod(const char *tarbase, int compression, bool sync)
 	method->write = tar_write;
 	method->get_current_pos = tar_get_current_pos;
 	method->get_file_size = tar_get_file_size;
+	method->get_file_name = tar_get_file_name;
+	method->compression = tar_compression;
 	method->close = tar_close;
 	method->sync = tar_sync;
 	method->existsfile = tar_existsfile;

src/bin/pg_basebackup/walmethods.h

@@ -52,6 +52,15 @@ struct WalWriteMethod
 	/* Return the size of a file, or -1 on failure. */
 	ssize_t		(*get_file_size) (const char *pathname);
 
+	/*
+	 * Return the name of the current file to work on, without the base
+	 * directory.  This is useful for logging.
+	 */
+	char	   *(*get_file_name) (const char *pathname, const char *temp_suffix);
+
+	/* Return the level of compression */
+	int			(*compression) (void);
+
 	/*
 	 * Write count number of bytes to the file, and return the number of bytes
 	 * actually written or -1 for error.