RFC: directory layout for notmuch git

RFC: directory layout for notmuch git

[David Bremner]

It turns out that if you have more than about 150k messages _and_ your
/tmp is on ext4, then the merge operation for notmuch-git can fail
(either hash collisions in the directory indexing, or running out of
inodes). There are few possible solutions. The simplest is just to
make /tmp tmpfs. Another solution is to implement a hashing scheme
like the patch that follows. This is not much work, but we have to
figure out what to do with existing repos. Do we write migration code
(considerably more complex than the hashing code itself), or just have
a flag day? This would involve everybody re-cloning from nmbug, but I
guess there are not that many active users. Other possibilities
include using the non-hashed layout when in "nmbug mode".

[PATCH] WIP/git: change internal directory layout

[David Bremner]

Add 4 layers of hashed directories in order to prevent large numbers
of subdirectories in any one directory.

Currently there is no migration strategy for old style notmuch-git /
nmbug repositories.
---
 notmuch-git.py   | 17 ++++++++++++-----
 test/T850-git.sh | 48 ++++++++++++++++++++++++------------------------
 test/test-lib.sh |  4 ++++
 3 files changed, 40 insertions(+), 29 deletions(-)

diff --git a/notmuch-git.py b/notmuch-git.py
index f188660c..61c5fe29 100644
--- a/notmuch-git.py
+++ b/notmuch-git.py
@@ -49,7 +49,7 @@ TAG_PREFIX = None
 
 _HEX_ESCAPE_REGEX = _re.compile('%[0-9A-F]{2}')
 _TAG_DIRECTORY = 'tags/'
-_TAG_FILE_REGEX = _re.compile(_TAG_DIRECTORY + '(?P<id>[^/]*)/(?P<tag>[^/]*)')
+_TAG_FILE_REGEX = _re.compile(_TAG_DIRECTORY + '([0-9a-f]{2}/){4}(?P<id>[^/]*)/(?P<tag>[^/]*)')
 
 # magic hash for Git (git hash-object -t blob /dev/null)
 _EMPTYBLOB = 'e69de29bb2d1d6434b8b29ae775ad8c2e48c5391'
@@ -265,7 +265,7 @@ def archive(treeish='HEAD', args=()):
     Each tag $tag for message with Message-Id $id is written to
     an empty file
 
-      tags/encode($id)/encode($tag)
+      tags/hash1(id)/hash2(id)/hash3(id)/hash4(id)/encode($id)/encode($tag)
 
     The encoding preserves alphanumerics, and the characters
     "+-_@=.:," (not the quotes).  All other octets are replaced with
@@ -821,7 +821,7 @@ def _clear_tags_for_message(index, id):
     Neither 'id' nor the tags in 'tags' should be encoded/escaped.
     """
 
-    dir = 'tags/{id}'.format(id=_hex_quote(string=id))
+    dir = _id_path(id)
 
     with _git(
             args=['ls-files', dir],
@@ -838,6 +838,14 @@ def _read_database_lastmod():
         (count,uuid,lastmod_str) = notmuch.stdout.readline().split()
         return (count,uuid,int(lastmod_str))
 
+def _id_path(id):
+    from hashlib import blake2b
+    hid=_hex_quote(string=id)
+    idhash = blake2b(hid.encode('utf8'), digest_size=4).hexdigest()
+    return 'tags/{dir1}/{dir2}/{dir3}/{dir4}/{hid}'.format(hid=hid,
+                                                            dir1=idhash[0:2],dir2=idhash[2:4],
+                                                            dir3=idhash[4:6],dir4=idhash[6:])
+
 def _index_tags_for_message(id, status, tags):
     """
     Update the Git index to either create or delete an empty file.
@@ -852,8 +860,7 @@ def _index_tags_for_message(id, status, tags):
         hash = '0000000000000000000000000000000000000000'
 
     for tag in tags:
-        path = 'tags/{id}/{tag}'.format(
-            id=_hex_quote(string=id), tag=_hex_quote(string=tag))
+        path = '{ipath}/{tag}'.format(ipath=_id_path(id),tag=_hex_quote(string=tag))
         yield '{mode} {hash}\t{path}\n'.format(mode=mode, hash=hash, path=path)
 
 
diff --git a/test/T850-git.sh b/test/T850-git.sh
index 7ea50939..dfff2369 100755
--- a/test/T850-git.sh
+++ b/test/T850-git.sh
@@ -40,10 +40,10 @@ notmuch tag -new-prefix::foo id:20091117190054.GU3165@dottiness.seas.harvard.edu
 test_begin_subtest "committing new prefix works with force"
 notmuch tag +new-prefix::foo id:20091117190054.GU3165@dottiness.seas.harvard.edu
 notmuch git -l debug -p 'new-prefix::' -C force-prefix.git commit --force
-git -C force-prefix.git ls-tree -r --name-only HEAD | xargs dirname | sort -u | sed s,tags/,id:, > OUTPUT
+git -C force-prefix.git ls-tree -r --name-only HEAD | xargs dirname | notmuch_git_sanitize | sort -u > OUTPUT
 notmuch tag -new-prefix::foo id:20091117190054.GU3165@dottiness.seas.harvard.edu
 cat <<EOF>EXPECTED
-id:20091117190054.GU3165@dottiness.seas.harvard.edu
+20091117190054.GU3165@dottiness.seas.harvard.edu
 EOF
 test_expect_equal_file_nonempty EXPECTED OUTPUT
 
@@ -62,8 +62,8 @@ test_expect_equal_file_nonempty EXPECTED OUTPUT
 
 test_begin_subtest "commit"
 notmuch git -C tags.git commit --force
-git -C tags.git ls-tree -r --name-only HEAD | xargs dirname | sort -u | sed s,tags/,id:, > OUTPUT
-notmuch search --output=messages '*' | sort > EXPECTED
+git -C tags.git ls-tree -r --name-only HEAD | xargs dirname | notmuch_git_sanitize | sort -u > OUTPUT
+notmuch search --output=messages '*' | sed s/^id:// | sort > EXPECTED
 test_expect_equal_file_nonempty EXPECTED OUTPUT
 
 test_begin_subtest "commit --force succeeds"
@@ -88,22 +88,22 @@ test_expect_equal_file_nonempty BEFORE AFTER
 test_begin_subtest "commit (incremental)"
 notmuch tag +test id:20091117190054.GU3165@dottiness.seas.harvard.edu
 notmuch git -C tags.git commit
-git -C tags.git ls-tree -r --name-only HEAD |
+git -C tags.git ls-tree -r --name-only HEAD | notmuch_git_sanitize | \
     grep 20091117190054 | sort > OUTPUT
 echo "--------------------------------------------------" >> OUTPUT
 notmuch tag -test id:20091117190054.GU3165@dottiness.seas.harvard.edu
 notmuch git -C tags.git commit
-git -C tags.git ls-tree -r --name-only HEAD |
+git -C tags.git ls-tree -r --name-only HEAD | notmuch_git_sanitize | \
     grep 20091117190054 | sort >> OUTPUT
 cat <<EOF > EXPECTED
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/inbox
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/signed
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/test
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/unread
+20091117190054.GU3165@dottiness.seas.harvard.edu/inbox
+20091117190054.GU3165@dottiness.seas.harvard.edu/signed
+20091117190054.GU3165@dottiness.seas.harvard.edu/test
+20091117190054.GU3165@dottiness.seas.harvard.edu/unread
 --------------------------------------------------
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/inbox
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/signed
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/unread
+20091117190054.GU3165@dottiness.seas.harvard.edu/inbox
+20091117190054.GU3165@dottiness.seas.harvard.edu/signed
+20091117190054.GU3165@dottiness.seas.harvard.edu/unread
 EOF
 test_expect_equal_file_nonempty EXPECTED OUTPUT
 
@@ -111,18 +111,18 @@ test_begin_subtest "commit (change prefix)"
 notmuch tag +test::one id:20091117190054.GU3165@dottiness.seas.harvard.edu
 notmuch git -C tags.git -p 'test::' commit --force
 git -C tags.git ls-tree -r --name-only HEAD |
-    grep 20091117190054 | sort > OUTPUT
+    grep 20091117190054 | notmuch_git_sanitize | sort > OUTPUT
 echo "--------------------------------------------------" >> OUTPUT
 notmuch tag -test::one id:20091117190054.GU3165@dottiness.seas.harvard.edu
 notmuch git -C tags.git commit --force
-git -C tags.git ls-tree -r --name-only HEAD |
+git -C tags.git ls-tree -r --name-only HEAD | notmuch_git_sanitize | \
     grep 20091117190054 | sort >> OUTPUT
 cat <<EOF > EXPECTED
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/one
+20091117190054.GU3165@dottiness.seas.harvard.edu/one
 --------------------------------------------------
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/inbox
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/signed
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/unread
+20091117190054.GU3165@dottiness.seas.harvard.edu/inbox
+20091117190054.GU3165@dottiness.seas.harvard.edu/signed
+20091117190054.GU3165@dottiness.seas.harvard.edu/unread
 EOF
 test_expect_equal_file_nonempty EXPECTED OUTPUT
 
@@ -151,12 +151,12 @@ test_expect_equal_file_nonempty BEFORE AFTER
 
 test_begin_subtest "archive"
 notmuch git -C tags.git archive | tar tf - | \
-    grep 20091117190054.GU3165@dottiness.seas.harvard.edu | sort > OUTPUT
+    grep 20091117190054.GU3165@dottiness.seas.harvard.edu | notmuch_git_sanitize | sort > OUTPUT
 cat <<EOF > EXPECTED
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/inbox
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/signed
-tags/20091117190054.GU3165@dottiness.seas.harvard.edu/unread
+20091117190054.GU3165@dottiness.seas.harvard.edu/
+20091117190054.GU3165@dottiness.seas.harvard.edu/inbox
+20091117190054.GU3165@dottiness.seas.harvard.edu/signed
+20091117190054.GU3165@dottiness.seas.harvard.edu/unread
 EOF
 notmuch git -C tags.git checkout
 test_expect_equal_file EXPECTED OUTPUT
diff --git a/test/test-lib.sh b/test/test-lib.sh
index 59b6079d..ad490293 100644
--- a/test/test-lib.sh
+++ b/test/test-lib.sh
@@ -545,6 +545,10 @@ notmuch_date_sanitize () {
 	-e 's/^Date: Fri, 05 Jan 2001 .*0000/Date: GENERATED_DATE/'
 }
 
+# remove redundant parts of notmuch-git internal paths
+notmuch_git_sanitize () {
+    sed 's,tags/\([0-9a-f]\{2\}/\)\{4\},,'
+}
 notmuch_uuid_sanitize () {
     sed 's/[0-9a-f]\{8\}-[0-9a-f]\{4\}-[0-9a-f]\{4\}-[0-9a-f]\{4\}-[0-9a-f]\{12\}/UUID/g'
 }
-- 
2.35.2