In pg_ctl, work around ERROR_SHARING_VIOLATION on the postmaster log file.

On Windows, we use CMD.EXE to redirect the postmaster's stdout/stderr
into a log file.  CMD.EXE will open that file with non-sharing-friendly
parameters, and the file will remain open for a short time after the
postmaster has removed postmaster.pid.  This can result in an
ERROR_SHARING_VIOLATION failure if we attempt to start a new postmaster
immediately with the same log file (e.g. during "pg_ctl restart").
This seems to explain intermittent buildfarm failures we've been seeing
on Windows machines.

To fix, just open and close the log file using our own pgwin32_open(),
which will wait if necessary to avoid the failure.  (Perhaps someday
we should stop using CMD.EXE, but that would be a far more complex
patch, and it doesn't seem worth the trouble ... yet.)

Back-patch to v12.  This only solves the problem when frontend fopen()
is redirected to pgwin32_fopen(), which has only been true since commit
0ba06e0b.  Hence, no point in back-patching further, unless we care
to back-patch that change too.

Diagnosis and patch by Alexander Lakhin (bug #16154).

Discussion: https://postgr.es/m/16154-1ccf0b537b24d5e0@postgresql.org

src/bin/pg_ctl/pg_ctl.c

@@ -519,8 +519,29 @@ start_postmaster(void)
 		comspec = "CMD";
 
 	if (log_file != NULL)
+	{
+		/*
+		 * First, touch the log file.  The main value of this is that if the
+		 * file is still locked by a previous postmaster run, we'll wait until
+		 * it comes free, instead of failing with ERROR_SHARING_VIOLATION.
+		 * (It'd be better to open the file in a sharing-friendly mode, but we
+		 * can't use CMD.EXE to do that, so work around it.  Note that the
+		 * previous postmaster will still have the file open for a short time
+		 * after removing postmaster.pid.)
+		 */
+		FILE	   *fd = fopen(log_file, "a");
+
+		if (fd == NULL)
+		{
+			write_stderr(_("%s: could not create log file \"%s\": %s\n"),
+						 progname, log_file, strerror(errno));
+			exit(1);
+		}
+		fclose(fd);
+
 		snprintf(cmd, MAXPGPATH, "\"%s\" /C \"\"%s\" %s%s < \"%s\" >> \"%s\" 2>&1\"",
 				 comspec, exec_path, pgdata_opt, post_opts, DEVNULL, log_file);
+	}
 	else
 		snprintf(cmd, MAXPGPATH, "\"%s\" /C \"\"%s\" %s%s < \"%s\" 2>&1\"",
 				 comspec, exec_path, pgdata_opt, post_opts, DEVNULL);