Dump currently only takes the read lock. Xapian can cope with some
changes while maintaining a read snapshot but with more changes it
fails. Currently notmuch just gives a xapian error.

To avoid this we take the write lock when dumping. This prevents other
notmuch processes from modifying the xapian database preventing this
error.

Discussion with Olly on irc indicates that this is currently the best
solution: in xapian trunk there may be better possibilities using
snapshots but they need to make it to a release and propogate out to
users before we can switch approach.

Finally, this breaks one use case: pipelines of the form

notmuch dump | ... | notmuch restore

According to Olly this is already very fragile: it will only work on
small databases. One of the tests relies on this behaviour so fix that
to store the dump rather than use a pipe.
---
I haven't tested this much: all tests pass (with the one fix) and it is simple.

Best wishes

Mark


 notmuch-dump.c       |    2 +-
 test/T150-tagging.sh |    4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/notmuch-dump.c b/notmuch-dump.c
index 887a208..9c6ad7f 100644
--- a/notmuch-dump.c
+++ b/notmuch-dump.c
@@ -212,7 +212,7 @@ notmuch_dump_command (notmuch_config_t *config, int argc, char *argv[])
     int ret;
 
     if (notmuch_database_open (notmuch_config_get_database_path (config),
-			       NOTMUCH_DATABASE_MODE_READ_ONLY, &notmuch))
+			       NOTMUCH_DATABASE_MODE_READ_WRITE, &notmuch))
 	return EXIT_FAILURE;
 
     char *output_file_name = NULL;
diff --git a/test/T150-tagging.sh b/test/T150-tagging.sh
index dc118f3..45471ac 100755
--- a/test/T150-tagging.sh
+++ b/test/T150-tagging.sh
@@ -247,8 +247,8 @@ ${TEST_DIRECTORY}/random-corpus --config-path=${NOTMUCH_CONFIG} \
 notmuch dump --format=batch-tag | sed 's/^.* -- /+common_tag -- /' | \
     sort > EXPECTED
 
-notmuch dump --format=batch-tag | sed 's/^.* -- /  -- /' | \
-    notmuch restore --format=batch-tag
+notmuch dump --format=batch-tag | sed 's/^.* -- /  -- /' > INTERMEDIATE_STEP
+notmuch restore --format=batch-tag < INTERMEDIATE_STEP
 
 notmuch tag --batch < EXPECTED
 
-- 
1.7.10.4