[PATCH v4 00/11] receive-pack: use ODB transactions to stage object writes

Greetings,

This patch series replaces direct usage of the `tmp_objdir` interfaces
in git-receive-pack(1) to instead use the `odb_transaction` interfaces
to create/manage a staging area to write objects to. The purpose of this
change is to get git-receive-pack(1) one step closer to being ODB
backend agnostic. For now, the object writes themselves are still
"files" backend specific due to being handled by the git-index-pack(1)
and git-unpack-objects(1) child processes. This will be tackled in a
separate series though.

Changes since V3:
  - Removed ugly line break in commit message to prevent eye strain.
  - `odb_transaction_begin()` now only sets the repository transaction
    on success.
  - `odb_transaction_env()` now bubbles up error when failing to create
    the temporary directory.

Changes since V2:
  - Clarified commit log reasoning for embedding
    `flush_loose_object_transaction()` logic in commit function.
  - Started printed some error messages on transaction errors.
  - Removed include statement.
  - Fixed transaction leak on `odb_transaction_commit()` error.

Changes since V1:
  - Adapted other "file" ODB transaction helpers to be more consistent
    with current naming scheme.
  - Removed redundant NULL transaction handling from
    `odb_transaction_files_begin()`.
  - `odb_transaction_begin()` now returns an error if there is already
    an inflight transaction pending instead of setting the `out` pointer
    to NULL.
  - Updated `odb_transaction_env()` to return an error code and append
    environment variables to a strvec provided as an argument.
  - Removed redundant setting of tmpdir environment variables for child
    processes after tmpdir has been migrated.
  - Split changes adding ODB transaction flags into a separate commit.
  - Consistently wire the ODB transaction throughout git-receive-pack
    code instead of reading it from `the_repository`.
  - Updated user facing error message.
  - Updated some comments to better document functions/flags.
  - Clarified some commit messages.
  - Fixed typos.

Thanks,
-Justin

Justin Tobler (11):
  object-file: rename files transaction prepare function
  object-file: rename files transaction fsync function
  object-file: embed transaction flush logic in commit function
  object-file: drop check for inflight transactions
  object-file: propagate files transaction errors
  odb/transaction: propagate begin errors
  odb/transaction: propagate commit errors
  odb/transaction: add transaction env interface
  odb/transaction: introduce ODB transaction flags
  builtin/receive-pack: drop redundant tmpdir env
  builtin/receive-pack: stage incoming objects via ODB transactions

 builtin/add.c            |   2 +-
 builtin/receive-pack.c   |  69 ++++++++---------
 builtin/unpack-objects.c |   2 +-
 builtin/update-index.c   |   2 +-
 cache-tree.c             |   7 +-
 object-file.c            | 161 +++++++++++++++++++++++++--------------
 object-file.h            |   8 +-
 odb/source-files.c       |   9 +--
 odb/source-inmemory.c    |   3 +-
 odb/source-loose.c       |   3 +-
 odb/source.h             |   9 ++-
 odb/transaction.c        |  33 ++++++--
 odb/transaction.h        |  59 +++++++++++---
 read-cache.c             |   7 +-
 14 files changed, 244 insertions(+), 130 deletions(-)

Range-diff against v3:
 1:  9c14b219ad =  1:  9c14b219ad object-file: rename files transaction prepare function
 2:  5703a9e93b =  2:  5703a9e93b object-file: rename files transaction fsync function
 3:  76204847f2 !  3:  70267741b0 object-file: embed transaction flush logic in commit function
    @@ Commit message
         subsequent commit, the transaction temporary directory is used to stage
         packfiles and not just loose objects anymore.
     
    -    Lift the helper function logic directly into
    -    `odb_transaction_files_commit()` to more accurately signal to readers
    -    the operation being performed.
    +    Lift the helper function logic into `odb_transaction_files_commit()` to
    +    more accurately signal to readers the operation being performed.
     
         Signed-off-by: Justin Tobler [off-list ref]
     
 4:  c97eb7763f =  4:  34cd3822c5 object-file: drop check for inflight transactions
 5:  1f3a1f7714 =  5:  240aa3475f object-file: propagate files transaction errors
 6:  09d13272d5 !  6:  0d91310fac odb/transaction: propagate begin errors
    @@ odb/transaction.c
      
     -	odb_source_begin_transaction(odb->sources, &odb->transaction);
     +	ret = odb_source_begin_transaction(odb->sources, out);
    -+	odb->transaction = *out;
    ++	if (!ret)
    ++		odb->transaction = *out;
      
     -	return odb->transaction;
     +	return ret;
 7:  12833d6773 =  7:  5e4680ed75 odb/transaction: propagate commit errors
 8:  f2586f2f34 !  8:  babcf6b156 odb/transaction: add transaction env interface
    @@ object-file.c: static int odb_transaction_files_commit(struct odb_transaction *b
     +{
     +	struct odb_transaction_files *transaction =
     +		container_of(base, struct odb_transaction_files, base);
    ++	int ret;
     +
    -+	odb_transaction_files_prepare(&transaction->base);
    -+	strvec_pushv(env, tmp_objdir_env(transaction->objdir));
    ++	ret = odb_transaction_files_prepare(&transaction->base);
    ++	if (!ret)
    ++		strvec_pushv(env, tmp_objdir_env(transaction->objdir));
     +
    -+	return 0;
    ++	return ret;
     +}
     +
      int odb_transaction_files_begin(struct odb_source *source,
 9:  9d082b5e47 !  9:  96f2a21eec odb/transaction: introduce ODB transaction flags
    @@ odb/transaction.c
      
     -	ret = odb_source_begin_transaction(odb->sources, out);
     +	ret = odb_source_begin_transaction(odb->sources, out, flags);
    - 	odb->transaction = *out;
    + 	if (!ret)
    + 		odb->transaction = *out;
      
    - 	return ret;
     
      ## odb/transaction.h ##
     @@
10:  e11d8a6676 = 10:  56718f1190 builtin/receive-pack: drop redundant tmpdir env
11:  fee57c2817 = 11:  5197a19fbf builtin/receive-pack: stage incoming objects via ODB transactions

base-commit: ab776a62a78576513ee121424adb19597fbb7613
-- 
2.55.0.122.gf85a7e6620