basic_archive.c

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/*-------------------------------------------------------------------------
 *
 * 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) 2022-2023, 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 "archive/archive_module.h"
#include "common/int.h"
#include "miscadmin.h"
#include "storage/copydir.h"
#include "storage/fd.h"
#include "utils/guc.h"
#include "utils/memutils.h"
 
PG_MODULE_MAGIC;
 
typedef struct BasicArchiveData
{
    MemoryContext context;
} BasicArchiveData;
 
static char *archive_directory = NULL;
 
static void basic_archive_startup(ArchiveModuleState *state);
static bool basic_archive_configured(ArchiveModuleState *state);
static bool basic_archive_file(ArchiveModuleState *state, 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);
static void basic_archive_shutdown(ArchiveModuleState *state);
 
static const ArchiveModuleCallbacks basic_archive_callbacks = {
    .startup_cb = basic_archive_startup,
    .check_configured_cb = basic_archive_configured,
    .archive_file_cb = basic_archive_file,
    .shutdown_cb = basic_archive_shutdown
};
 
/*
 * _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");
}
 
/*
 * _PG_archive_module_init
 *
 * Returns the module's archiving callbacks.
 */
const ArchiveModuleCallbacks *
_PG_archive_module_init(void)
{
    return &basic_archive_callbacks;
}
 
/*
 * basic_archive_startup
 *
 * Creates the module's memory context.
 */
void
basic_archive_startup(ArchiveModuleState *state)
{
    BasicArchiveData *data;
 
    data = (BasicArchiveData *) MemoryContextAllocZero(TopMemoryContext,
                                                       sizeof(BasicArchiveData));
    data->context = AllocSetContextCreate(TopMemoryContext,
                                          "basic_archive",
                                          ALLOCSET_DEFAULT_SIZES);
    state->private_data = (void *) data;
}
 
/*
 * 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(ArchiveModuleState *state)
{
    return archive_directory != NULL && archive_directory[0] != '\0';
}
 
/*
 * basic_archive_file
 *
 * Archives one file.
 */
static bool
basic_archive_file(ArchiveModuleState *state, const char *file, const char *path)
{
    sigjmp_buf  local_sigjmp_buf;
    MemoryContext oldcontext;
    BasicArchiveData *data = (BasicArchiveData *) state->private_data;
    MemoryContext basic_archive_context = data->context;
 
    /*
     * 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;          /* milliseconds */
 
    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 / 1000), &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(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;
}
 
/*
 * basic_archive_shutdown
 *
 * Frees our allocated state.
 */
static void
basic_archive_shutdown(ArchiveModuleState *state)
{
    BasicArchiveData *data = (BasicArchiveData *) state->private_data;
    MemoryContext basic_archive_context;
 
    /*
     * If we didn't get to storing the pointer to our allocated state, we don't
     * have anything to clean up.
     */
    if (data == NULL)
        return;
 
    basic_archive_context = data->context;
    Assert(CurrentMemoryContext != basic_archive_context);
 
    if (MemoryContextIsValid(basic_archive_context))
        MemoryContextDelete(basic_archive_context);
    data->context = NULL;
 
    /*
     * Finally, free the state.
     */
    pfree(data);
    state->private_data = NULL;
}