Rework internal command generation of pg_rewind

pg_rewind generates and executes internally up to two commands to work
on the target cluster, depending on the options given by its caller:
- postgres -C to retrieve the value of restore_command, when using
-c/--restore-target-wal.
- postgres --single to enforce recovery once and get the target cluster
in a clean shutdown state.

Both commands have been applying incorrect quoting rules, which could
lead to failures when for example using a target data directory with
unexpected characters like CRLFs.  Those commands are now generated with
PQExpBuffer, making use of string_utils.h to quote those commands as
they should.  We may extend those commands in the future with more
options, so this makes any upcoming additions easier.

This is arguably a bug fix, but nobody has complained about the existing
code being a problem either, so no backpatch is done.

Extracted from a larger patch by the same author.

Author: Gunnar "Nick" Bluth
Discussion: https://postgr.es/m/7c59265d-ac50-b0aa-ca1e-65e8bd27642a@pro-open.de

diff --git a/src/bin/pg_rewind/pg_rewind.c b/src/bin/pg_rewind/pg_rewind.c
index efb82a403416588a2df47645da8a5c134df73806..b39b5c1aacc961a2a60586198c4bb0b68b9dd747 100644 (file)
--- a/src/bin/pg_rewind/pg_rewind.c
+++ b/src/bin/pg_rewind/pg_rewind.c
@@ -23,6 +23,7 @@
 #include "common/restricted_token.h"
 #include "common/string.h"
 #include "fe_utils/recovery_gen.h"
+#include "fe_utils/string_utils.h"
 #include "file_ops.h"
 #include "filemap.h"
 #include "getopt_long.h"
@@ -1016,8 +1017,8 @@ getRestoreCommand(const char *argv0)
 {
    int         rc;
    char        postgres_exec_path[MAXPGPATH],
-               postgres_cmd[MAXPGPATH],
                cmd_output[MAXPGPATH];
+   PQExpBuffer postgres_cmd;
 
    if (!restore_wal)
        return;
@@ -1051,11 +1052,19 @@ getRestoreCommand(const char *argv0)
     * Build a command able to retrieve the value of GUC parameter
     * restore_command, if set.
     */
-   snprintf(postgres_cmd, sizeof(postgres_cmd),
-            "\"%s\" -D \"%s\" -C restore_command",
-            postgres_exec_path, datadir_target);
+   postgres_cmd = createPQExpBuffer();
 
-   if (!pipe_read_line(postgres_cmd, cmd_output, sizeof(cmd_output)))
+   /* path to postgres, properly quoted */
+   appendShellString(postgres_cmd, postgres_exec_path);
+
+   /* add -D switch, with properly quoted data directory */
+   appendPQExpBufferStr(postgres_cmd, " -D ");
+   appendShellString(postgres_cmd, datadir_target);
+
+   /* add -C switch, for restore_command */
+   appendPQExpBufferStr(postgres_cmd, " -C restore_command");
+
+   if (!pipe_read_line(postgres_cmd->data, cmd_output, sizeof(cmd_output)))
        exit(1);
 
    (void) pg_strip_crlf(cmd_output);
@@ -1067,6 +1076,8 @@ getRestoreCommand(const char *argv0)
 
    pg_log_debug("using for rewind restore_command = \'%s\'",
                 restore_command);
+
+   destroyPQExpBuffer(postgres_cmd);
 }
 
 
@@ -1080,7 +1091,7 @@ ensureCleanShutdown(const char *argv0)
    int         ret;
 #define MAXCMDLEN (2 * MAXPGPATH)
    char        exec_path[MAXPGPATH];
-   char        cmd[MAXCMDLEN];
+   PQExpBuffer postgres_cmd;
 
    /* locate postgres binary */
    if ((ret = find_other_exec(argv0, "postgres",
@@ -1119,14 +1130,26 @@ ensureCleanShutdown(const char *argv0)
     * fsync here.  This makes the recovery faster, and the target data folder
     * is synced at the end anyway.
     */
-   snprintf(cmd, MAXCMDLEN, "\"%s\" --single -F -D \"%s\" template1 < \"%s\"",
-            exec_path, datadir_target, DEVNULL);
+   postgres_cmd = createPQExpBuffer();
 
-   if (system(cmd) != 0)
+   /* path to postgres, properly quoted */
+   appendShellString(postgres_cmd, exec_path);
+
+   /* add set of options with properly quoted data directory */
+   appendPQExpBufferStr(postgres_cmd, " --single -F -D ");
+   appendShellString(postgres_cmd, datadir_target);
+
+   /* finish with the database name, and a properly quoted redirection */
+   appendPQExpBufferStr(postgres_cmd, " template1 < ");
+   appendShellString(postgres_cmd, DEVNULL);
+
+   if (system(postgres_cmd->data) != 0)
    {
        pg_log_error("postgres single-user mode in target cluster failed");
-       pg_fatal("Command was: %s", cmd);
+       pg_fatal("Command was: %s", postgres_cmd->data);
    }
+
+   destroyPQExpBuffer(postgres_cmd);
 }
 
 static void