basebackup_to_shell.c

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/*-------------------------------------------------------------------------
 *
 * basebackup_to_shell.c
 *    target base backup files to a shell command
 *
 * Copyright (c) 2016-2025, PostgreSQL Global Development Group
 *
 *    contrib/basebackup_to_shell/basebackup_to_shell.c
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/xact.h"
#include "backup/basebackup_target.h"
#include "common/percentrepl.h"
#include "miscadmin.h"
#include "storage/fd.h"
#include "utils/acl.h"
#include "utils/guc.h"
 
PG_MODULE_MAGIC;
 
typedef struct bbsink_shell
{
    /* Common information for all types of sink. */
    bbsink      base;
 
    /* User-supplied target detail string. */
    char       *target_detail;
 
    /* Shell command pattern being used for this backup. */
    char       *shell_command;
 
    /* The command that is currently running. */
    char       *current_command;
 
    /* Pipe to the running command. */
    FILE       *pipe;
} bbsink_shell;
 
static void *shell_check_detail(char *target, char *target_detail);
static bbsink *shell_get_sink(bbsink *next_sink, void *detail_arg);
 
static void bbsink_shell_begin_archive(bbsink *sink,
                                       const char *archive_name);
static void bbsink_shell_archive_contents(bbsink *sink, size_t len);
static void bbsink_shell_end_archive(bbsink *sink);
static void bbsink_shell_begin_manifest(bbsink *sink);
static void bbsink_shell_manifest_contents(bbsink *sink, size_t len);
static void bbsink_shell_end_manifest(bbsink *sink);
 
static const bbsink_ops bbsink_shell_ops = {
    .begin_backup = bbsink_forward_begin_backup,
    .begin_archive = bbsink_shell_begin_archive,
    .archive_contents = bbsink_shell_archive_contents,
    .end_archive = bbsink_shell_end_archive,
    .begin_manifest = bbsink_shell_begin_manifest,
    .manifest_contents = bbsink_shell_manifest_contents,
    .end_manifest = bbsink_shell_end_manifest,
    .end_backup = bbsink_forward_end_backup,
    .cleanup = bbsink_forward_cleanup
};
 
static char *shell_command = "";
static char *shell_required_role = "";
 
void
_PG_init(void)
{
    DefineCustomStringVariable("basebackup_to_shell.command",
                               "Shell command to be executed for each backup file.",
                               NULL,
                               &shell_command,
                               "",
                               PGC_SIGHUP,
                               0,
                               NULL, NULL, NULL);
 
    DefineCustomStringVariable("basebackup_to_shell.required_role",
                               "Backup user must be a member of this role to use shell backup target.",
                               NULL,
                               &shell_required_role,
                               "",
                               PGC_SIGHUP,
                               0,
                               NULL, NULL, NULL);
 
    MarkGUCPrefixReserved("basebackup_to_shell");
 
    BaseBackupAddTarget("shell", shell_check_detail, shell_get_sink);
}
 
/*
 * We choose to defer sanity checking until shell_get_sink(), and so
 * just pass the target detail through without doing anything. However, we do
 * permissions checks here, before any real work has been done.
 */
static void *
shell_check_detail(char *target, char *target_detail)
{
    if (shell_required_role[0] != '\0')
    {
        Oid         roleid;
 
        StartTransactionCommand();
        roleid = get_role_oid(shell_required_role, true);
        if (!has_privs_of_role(GetUserId(), roleid))
            ereport(ERROR,
                    (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                     errmsg("permission denied to use basebackup_to_shell")));
        CommitTransactionCommand();
    }
 
    return target_detail;
}
 
/*
 * Set up a bbsink to implement this base backup target.
 *
 * This is also a convenient place to sanity check that a target detail was
 * given if and only if %d is present.
 */
static bbsink *
shell_get_sink(bbsink *next_sink, void *detail_arg)
{
    bbsink_shell *sink;
    bool        has_detail_escape = false;
    char       *c;
 
    /*
     * Set up the bbsink.
     *
     * We remember the current value of basebackup_to_shell.shell_command to
     * be certain that it can't change under us during the backup.
     */
    sink = palloc0(sizeof(bbsink_shell));
    *((const bbsink_ops **) &sink->base.bbs_ops) = &bbsink_shell_ops;
    sink->base.bbs_next = next_sink;
    sink->target_detail = detail_arg;
    sink->shell_command = pstrdup(shell_command);
 
    /* Reject an empty shell command. */
    if (sink->shell_command[0] == '\0')
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                errmsg("shell command for backup is not configured"));
 
    /* Determine whether the shell command we're using contains %d. */
    for (c = sink->shell_command; *c != '\0'; ++c)
    {
        if (c[0] == '%' && c[1] != '\0')
        {
            if (c[1] == 'd')
                has_detail_escape = true;
            ++c;
        }
    }
 
    /* There should be a target detail if %d was used, and not otherwise. */
    if (has_detail_escape && sink->target_detail == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("a target detail is required because the configured command includes %%d"),
                 errhint("Try \"pg_basebackup --target shell:DETAIL ...\"")));
    else if (!has_detail_escape && sink->target_detail != NULL)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("a target detail is not permitted because the configured command does not include %%d")));
 
    /*
     * Since we're passing the string provided by the user to popen(), it will
     * be interpreted by the shell, which is a potential security
     * vulnerability, since the user invoking this module is not necessarily a
     * superuser. To stay out of trouble, we must disallow any shell
     * metacharacters here; to be conservative and keep things simple, we
     * allow only alphanumerics.
     */
    if (sink->target_detail != NULL)
    {
        char       *d;
        bool        scary = false;
 
        for (d = sink->target_detail; *d != '\0'; ++d)
        {
            if (*d >= 'a' && *d <= 'z')
                continue;
            if (*d >= 'A' && *d <= 'Z')
                continue;
            if (*d >= '0' && *d <= '9')
                continue;
            scary = true;
            break;
        }
 
        if (scary)
            ereport(ERROR,
                    errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                    errmsg("target detail must contain only alphanumeric characters"));
    }
 
    return &sink->base;
}
 
/*
 * Construct the exact shell command that we're actually going to run,
 * making substitutions as appropriate for escape sequences.
 */
static char *
shell_construct_command(const char *base_command, const char *filename,
                        const char *target_detail)
{
    return replace_percent_placeholders(base_command, "basebackup_to_shell.command",
                                        "df", target_detail, filename);
}
 
/*
 * Finish executing the shell command once all data has been written.
 */
static void
shell_finish_command(bbsink_shell *sink)
{
    int         pclose_rc;
 
    /* There should be a command running. */
    Assert(sink->current_command != NULL);
    Assert(sink->pipe != NULL);
 
    /* Close down the pipe we opened. */
    pclose_rc = ClosePipeStream(sink->pipe);
    if (pclose_rc == -1)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not close pipe to external command: %m")));
    else if (pclose_rc != 0)
    {
        ereport(ERROR,
                (errcode(ERRCODE_EXTERNAL_ROUTINE_EXCEPTION),
                 errmsg("shell command \"%s\" failed",
                        sink->current_command),
                 errdetail_internal("%s", wait_result_to_str(pclose_rc))));
    }
 
    /* Clean up. */
    sink->pipe = NULL;
    pfree(sink->current_command);
    sink->current_command = NULL;
}
 
/*
 * Start up the shell command, substituting %f in for the current filename.
 */
static void
shell_run_command(bbsink_shell *sink, const char *filename)
{
    /* There should not be anything already running. */
    Assert(sink->current_command == NULL);
    Assert(sink->pipe == NULL);
 
    /* Construct a suitable command. */
    sink->current_command = shell_construct_command(sink->shell_command,
                                                    filename,
                                                    sink->target_detail);
 
    /* Run it. */
    sink->pipe = OpenPipeStream(sink->current_command, PG_BINARY_W);
    if (sink->pipe == NULL)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not execute command \"%s\": %m",
                        sink->current_command)));
}
 
/*
 * Send accumulated data to the running shell command.
 */
static void
shell_send_data(bbsink_shell *sink, size_t len)
{
    /* There should be a command running. */
    Assert(sink->current_command != NULL);
    Assert(sink->pipe != NULL);
 
    /* Try to write the data. */
    if (fwrite(sink->base.bbs_buffer, len, 1, sink->pipe) != 1 ||
        ferror(sink->pipe))
    {
        if (errno == EPIPE)
        {
            /*
             * The error we're about to throw would shut down the command
             * anyway, but we may get a more meaningful error message by doing
             * this. If not, we'll fall through to the generic error below.
             */
            shell_finish_command(sink);
            errno = EPIPE;
        }
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not write to shell backup program: %m")));
    }
}
 
/*
 * At start of archive, start up the shell command and forward to next sink.
 */
static void
bbsink_shell_begin_archive(bbsink *sink, const char *archive_name)
{
    bbsink_shell *mysink = (bbsink_shell *) sink;
 
    shell_run_command(mysink, archive_name);
    bbsink_forward_begin_archive(sink, archive_name);
}
 
/*
 * Send archive contents to command's stdin and forward to next sink.
 */
static void
bbsink_shell_archive_contents(bbsink *sink, size_t len)
{
    bbsink_shell *mysink = (bbsink_shell *) sink;
 
    shell_send_data(mysink, len);
    bbsink_forward_archive_contents(sink, len);
}
 
/*
 * At end of archive, shut down the shell command and forward to next sink.
 */
static void
bbsink_shell_end_archive(bbsink *sink)
{
    bbsink_shell *mysink = (bbsink_shell *) sink;
 
    shell_finish_command(mysink);
    bbsink_forward_end_archive(sink);
}
 
/*
 * At start of manifest, start up the shell command and forward to next sink.
 */
static void
bbsink_shell_begin_manifest(bbsink *sink)
{
    bbsink_shell *mysink = (bbsink_shell *) sink;
 
    shell_run_command(mysink, "backup_manifest");
    bbsink_forward_begin_manifest(sink);
}
 
/*
 * Send manifest contents to command's stdin and forward to next sink.
 */
static void
bbsink_shell_manifest_contents(bbsink *sink, size_t len)
{
    bbsink_shell *mysink = (bbsink_shell *) sink;
 
    shell_send_data(mysink, len);
    bbsink_forward_manifest_contents(sink, len);
}
 
/*
 * At end of manifest, shut down the shell command and forward to next sink.
 */
static void
bbsink_shell_end_manifest(bbsink *sink)
{
    bbsink_shell *mysink = (bbsink_shell *) sink;
 
    shell_finish_command(mysink);
    bbsink_forward_end_manifest(sink);
}