postgresql/contrib/basic_archive/basic_archive.c

/*-------------------------------------------------------------------------
 *
 * basic_archive.c
 *
 * This file demonstrates a basic archive library implementation that is
 * roughly equivalent to the following shell command:
 *
 * 		test ! -f /path/to/dest && cp /path/to/src /path/to/dest
 *
 * One notable difference between this module and the shell command above
 * is that this module first copies the file to a temporary destination,
 * syncs it to disk, and then durably moves it to the final destination.
 *
 * Another notable difference is that if /path/to/dest already exists
 * but has contents identical to /path/to/src, archiving will succeed,
 * whereas the command shown above would fail. This prevents problems if
 * a file is successfully archived and then the system crashes before
 * a durable record of the success has been made.
 *
 * Copyright (c) 2022, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *	  contrib/basic_archive/basic_archive.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <sys/stat.h>
#include <sys/time.h>
#include <unistd.h>

#include "common/int.h"
#include "miscadmin.h"
#include "postmaster/pgarch.h"
#include "storage/copydir.h"
#include "storage/fd.h"
#include "utils/guc.h"
#include "utils/memutils.h"

PG_MODULE_MAGIC;

void _PG_init(void);
void _PG_archive_module_init(ArchiveModuleCallbacks *cb);

static char *archive_directory = NULL;
static MemoryContext basic_archive_context;

static bool basic_archive_configured(void);
static bool basic_archive_file(const char *file, const char *path);
static void basic_archive_file_internal(const char *file, const char *path);
static bool check_archive_directory(char **newval, void **extra, GucSource source);
static bool compare_files(const char *file1, const char *file2);

/*
 * _PG_init
 *
 * Defines the module's GUC.
 */
void
_PG_init(void)
{
	DefineCustomStringVariable("basic_archive.archive_directory",
							   gettext_noop("Archive file destination directory."),
							   NULL,
							   &archive_directory,
							   "",
							   PGC_SIGHUP,
							   0,
							   check_archive_directory, NULL, NULL);

	MarkGUCPrefixReserved("basic_archive");

	basic_archive_context = AllocSetContextCreate(TopMemoryContext,
												  "basic_archive",
												  ALLOCSET_DEFAULT_SIZES);
}

/*
 * _PG_archive_module_init
 *
 * Returns the module's archiving callbacks.
 */
void
_PG_archive_module_init(ArchiveModuleCallbacks *cb)
{
	AssertVariableIsOfType(&_PG_archive_module_init, ArchiveModuleInit);

	cb->check_configured_cb = basic_archive_configured;
	cb->archive_file_cb = basic_archive_file;
}

/*
 * check_archive_directory
 *
 * Checks that the provided archive directory exists.
 */
static bool
check_archive_directory(char **newval, void **extra, GucSource source)
{
	struct stat st;

	/*
	 * The default value is an empty string, so we have to accept that value.
	 * Our check_configured callback also checks for this and prevents archiving
	 * from proceeding if it is still empty.
	 */
	if (*newval == NULL || *newval[0] == '\0')
		return true;

	/*
	 * Make sure the file paths won't be too long.  The docs indicate that the
	 * file names to be archived can be up to 64 characters long.
	 */
	if (strlen(*newval) + 64 + 2 >= MAXPGPATH)
	{
		GUC_check_errdetail("archive directory too long");
		return false;
	}

	/*
	 * Do a basic sanity check that the specified archive directory exists.  It
	 * could be removed at some point in the future, so we still need to be
	 * prepared for it not to exist in the actual archiving logic.
	 */
	if (stat(*newval, &st) != 0 || !S_ISDIR(st.st_mode))
	{
		GUC_check_errdetail("specified archive directory does not exist");
		return false;
	}

	return true;
}

/*
 * basic_archive_configured
 *
 * Checks that archive_directory is not blank.
 */
static bool
basic_archive_configured(void)
{
	return archive_directory != NULL && archive_directory[0] != '\0';
}

/*
 * basic_archive_file
 *
 * Archives one file.
 */
static bool
basic_archive_file(const char *file, const char *path)
{
	sigjmp_buf	local_sigjmp_buf;
	MemoryContext oldcontext;

	/*
	 * We run basic_archive_file_internal() in our own memory context so that we
	 * can easily reset it during error recovery (thus avoiding memory leaks).
	 */
	oldcontext = MemoryContextSwitchTo(basic_archive_context);

	/*
	 * Since the archiver operates at the bottom of the exception stack, ERRORs
	 * turn into FATALs and cause the archiver process to restart.  However,
	 * using ereport(ERROR, ...) when there are problems is easy to code and
	 * maintain.  Therefore, we create our own exception handler to catch ERRORs
	 * and return false instead of restarting the archiver whenever there is a
	 * failure.
	 */
	if (sigsetjmp(local_sigjmp_buf, 1) != 0)
	{
		/* Since not using PG_TRY, must reset error stack by hand */
		error_context_stack = NULL;

		/* Prevent interrupts while cleaning up */
		HOLD_INTERRUPTS();

		/* Report the error and clear ErrorContext for next time */
		EmitErrorReport();
		FlushErrorState();

		/* Close any files left open by copy_file() or compare_files() */
		AtEOSubXact_Files(false, InvalidSubTransactionId, InvalidSubTransactionId);

		/* Reset our memory context and switch back to the original one */
		MemoryContextSwitchTo(oldcontext);
		MemoryContextReset(basic_archive_context);

		/* Remove our exception handler */
		PG_exception_stack = NULL;

		/* Now we can allow interrupts again */
		RESUME_INTERRUPTS();

		/* Report failure so that the archiver retries this file */
		return false;
	}

	/* Enable our exception handler */
	PG_exception_stack = &local_sigjmp_buf;

	/* Archive the file! */
	basic_archive_file_internal(file, path);

	/* Remove our exception handler */
	PG_exception_stack = NULL;

	/* Reset our memory context and switch back to the original one */
	MemoryContextSwitchTo(oldcontext);
	MemoryContextReset(basic_archive_context);

	return true;
}

static void
basic_archive_file_internal(const char *file, const char *path)
{
	char		destination[MAXPGPATH];
	char		temp[MAXPGPATH + 256];
	struct stat st;
	struct timeval tv;
	uint64		epoch;

	ereport(DEBUG3,
			(errmsg("archiving \"%s\" via basic_archive", file)));

	snprintf(destination, MAXPGPATH, "%s/%s", archive_directory, file);

	/*
	 * First, check if the file has already been archived.  If it already exists
	 * and has the same contents as the file we're trying to archive, we can
	 * return success (after ensuring the file is persisted to disk). This
	 * scenario is possible if the server crashed after archiving the file but
	 * before renaming its .ready file to .done.
	 *
	 * If the archive file already exists but has different contents, something
	 * might be wrong, so we just fail.
	 */
	if (stat(destination, &st) == 0)
	{
		if (compare_files(path, destination))
		{
			ereport(DEBUG3,
					(errmsg("archive file \"%s\" already exists with identical contents",
							destination)));

			fsync_fname(destination, false);
			fsync_fname(archive_directory, true);

			return;
		}

		ereport(ERROR,
				(errmsg("archive file \"%s\" already exists", destination)));
	}
	else if (errno != ENOENT)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not stat file \"%s\": %m", destination)));

	/*
	 * Pick a sufficiently unique name for the temporary file so that a
	 * collision is unlikely.  This helps avoid problems in case a temporary
	 * file was left around after a crash or another server happens to be
	 * archiving to the same directory.
	 */
	gettimeofday(&tv, NULL);
	if (pg_mul_u64_overflow((uint64) 1000, (uint64) tv.tv_sec, &epoch) ||
		pg_add_u64_overflow(epoch, (uint64) tv.tv_usec, &epoch))
		elog(ERROR, "could not generate temporary file name for archiving");

	snprintf(temp, sizeof(temp), "%s/%s.%s.%d." UINT64_FORMAT,
			 archive_directory, "archtemp", file, MyProcPid, epoch);

	/*
	 * Copy the file to its temporary destination.  Note that this will fail if
	 * temp already exists.
	 */
	copy_file(unconstify(char *, path), temp);

	/*
	 * Sync the temporary file to disk and move it to its final destination.
	 * Note that this will overwrite any existing file, but this is only
	 * possible if someone else created the file since the stat() above.
	 */
	(void) durable_rename(temp, destination, ERROR);

	ereport(DEBUG1,
			(errmsg("archived \"%s\" via basic_archive", file)));
}

/*
 * compare_files
 *
 * Returns whether the contents of the files are the same.
 */
static bool
compare_files(const char *file1, const char *file2)
{
#define CMP_BUF_SIZE (4096)
	char		buf1[CMP_BUF_SIZE];
	char		buf2[CMP_BUF_SIZE];
	int			fd1;
	int			fd2;
	bool		ret = true;

	fd1 = OpenTransientFile(file1, O_RDONLY | PG_BINARY);
	if (fd1 < 0)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not open file \"%s\": %m", file1)));

	fd2 = OpenTransientFile(file2, O_RDONLY | PG_BINARY);
	if (fd2 < 0)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not open file \"%s\": %m", file2)));

	for (;;)
	{
		int		nbytes = 0;
		int		buf1_len = 0;
		int		buf2_len = 0;

		while (buf1_len < CMP_BUF_SIZE)
		{
			nbytes = read(fd1, buf1 + buf1_len, CMP_BUF_SIZE - buf1_len);
			if (nbytes < 0)
				ereport(ERROR,
						(errcode_for_file_access(),
						 errmsg("could not read file \"%s\": %m", file1)));
			else if (nbytes == 0)
				break;

			buf1_len += nbytes;
		}

		while (buf2_len < CMP_BUF_SIZE)
		{
			nbytes = read(fd2, buf2 + buf2_len, CMP_BUF_SIZE - buf2_len);
			if (nbytes < 0)
				ereport(ERROR,
						(errcode_for_file_access(),
						 errmsg("could not read file \"%s\": %m", file2)));
			else if (nbytes == 0)
				break;

			buf2_len += nbytes;
		}

		if (buf1_len != buf2_len || memcmp(buf1, buf2, buf1_len) != 0)
		{
			ret = false;
			break;
		}
		else if (buf1_len == 0)
			break;
	}

	if (CloseTransientFile(fd1) != 0)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not close file \"%s\": %m", file1)));

	if (CloseTransientFile(fd2) != 0)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not close file \"%s\": %m", file2)));

	return ret;
}