webmap-import: Add a cache layer and store the source file's last modification time.

That way we can avoid the expensive unpack+import when the source
file(s) have not been updated since the last run.  The check can be
bypassed with a new flag `--force`.

We use a sequence for the FID:s (primary key) and a UNIQUE constraint on
triplets (source_path, archive_member, layername) as GDAL doesn't
support multicolumns primary keys.

To avoid races between the stat(2) calls, gdal.OpenEx() and updates via
`webmap-download` runs we place a shared lock on the downloaded files.
One could resort to some tricks to eliminate the race between the first
two, but there is also some value in having consistency during the
entire execution of the script (a single source file can be used by
multiple layers for instance, and it makes sense to use the very same
file for all layers in that case).

We also intersperse dso.FlushCache() calls between _importSource() calls
in order to force the PG driver to call EndCopy() to detect errors and
trigger a rollback when _importSource() fails.

1 files changed, 3 insertions, 4 deletions

diff --git a/webmap-download b/webmap-download
index edb624e..05aa2c4 100755
--- a/webmap-download
+++ b/webmap-download
@@ -44,12 +44,11 @@ import argparse
 import itertools
 from pathlib import Path
 from email.utils import parsedate_to_datetime, formatdate
-from hashlib import sha256
 from typing import Optional, NoReturn, Never
 import requests
 
 import common
-from common import BadConfiguration
+from common import BadConfiguration, getSourcePathLockFileName
 
 def download_trystream(url : str, **kwargs) -> requests.Response:
     """GET a url, trying a number of times. Return immediately after the
@@ -309,9 +308,9 @@ def main() -> NoReturn:
             # place an exclusive lock on a lockfile as the destination can be used by other layers
             # hence might be updated in parallel
             if lockdir_fd is not None:
-                lockfile = sha256(dest.encode('utf-8')).hexdigest() + '.lck'
+                lockfile = getSourcePathLockFileName(dest)
                 # use O_TRUNC to bump lockfile's mtime
-                lock_fd = os.open(lockfile, O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, mode=0o644,
+                lock_fd = os.open(lockfile, O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, mode=0o664,
                                   dir_fd=lockdir_fd)
             try:
                 if lockdir_fd is not None: