Fix error message on short read of pg_control

Instead of saying "error: success", indicate that we got a working read
but it was too short.

Author: mhagander

src/backend/access/transam/xlog.c

@@ -4311,6 +4311,7 @@ ReadControlFile(void)
 {
 	pg_crc32c	crc;
 	int			fd;
+	int			r;
 
 	/*
 	 * Read data...
@@ -4324,10 +4325,17 @@ ReadControlFile(void)
 				 errmsg("could not open control file \"%s\": %m",
 						XLOG_CONTROL_FILE)));
 
-	if (read(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
-		ereport(PANIC,
-				(errcode_for_file_access(),
-				 errmsg("could not read from control file: %m")));
+	r = read(fd, ControlFile, sizeof(ControlFileData));
+	if (r != sizeof(ControlFileData))
+	{
+		if (r < 0)
+			ereport(PANIC,
+					(errcode_for_file_access(),
+					 errmsg("could not read from control file: %m")));
+		else
+			ereport(PANIC,
+					 (errmsg("could not read from control file: read %d bytes, expected %d", r, (int) sizeof(ControlFileData))));
+	}
 
 	close(fd);
 

src/common/controldata_utils.c

@@ -41,6 +41,7 @@ get_controlfile(char *DataDir, const char *progname)
 	int			fd;
 	char		ControlFilePath[MAXPGPATH];
 	pg_crc32c	crc;
+	int			r;
 
 	ControlFile = palloc(sizeof(ControlFileData));
 	snprintf(ControlFilePath, MAXPGPATH, "%s/global/pg_control", DataDir);
@@ -59,18 +60,34 @@ get_controlfile(char *DataDir, const char *progname)
 	}
 #endif
 
-	if (read(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
+	r = read(fd, ControlFile, sizeof(ControlFileData));
+	if (r != sizeof(ControlFileData))
+	{
+		if (r < 0)
 #ifndef FRONTEND
-		ereport(ERROR,
-				(errcode_for_file_access(),
-				 errmsg("could not read file \"%s\": %m", ControlFilePath)));
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not read file \"%s\": %m", ControlFilePath)));
 #else
-	{
-		fprintf(stderr, _("%s: could not read file \"%s\": %s\n"),
-				progname, ControlFilePath, strerror(errno));
-		exit(EXIT_FAILURE);
-	}
+		{
+			fprintf(stderr, _("%s: could not read file \"%s\": %s\n"),
+					progname, ControlFilePath, strerror(errno));
+			exit(EXIT_FAILURE);
+		}
 #endif
+		else
+#ifndef FRONTEND
+			ereport(ERROR,
+					(errmsg("could not read file \"%s\": read %d bytes, expected %d",
+							ControlFilePath, r, (int) sizeof(ControlFileData))));
+#else
+		{
+			fprintf(stderr, _("%s: could not read file \"%s\": read %d bytes, expected %d\n"),
+					progname, ControlFilePath, r, (int) sizeof(ControlFileData));
+			exit(EXIT_FAILURE);
+		}
+#endif
+	}
 
 	close(fd);