fsck: Provide a function to parse fsck message IDs

This function will be used in the next commits to allow the user to
ask fsck to handle specific problems differently, e.g. demoting certain
errors to warnings. It has to handle partial strings because we would
like to be able to parse, say, 'missing-email,missing-tagger-entry'
command lines.

To make the parsing robust, we generate strings from the enum keys, and
using these keys, we will map lower-case, dash-separated strings values
to the corresponding enum values.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Offer a function to demote fsck errors to warnings

There are legacy repositories out there whose older commits and tags
have issues that prevent pushing them when 'receive.fsckObjects' is set.
One real-life example is a commit object that has been hand-crafted to
list two authors.

Often, it is not possible to fix those issues without disrupting the
work with said repositories, yet it is still desirable to perform checks
by setting `receive.fsckObjects = true`. This commit is the first step
to allow demoting specific fsck issues to mere warnings.

The function added by this commit parses a list of settings in the form:

	missing-email=warn,bad-name=warn,...

Unfortunately, the FSCK_WARN/FSCK_ERROR flag is only really heeded by
git fsck so far, but other call paths (e.g. git index-pack --strict)
error out *always* no matter what type was specified. Therefore, we
need to take extra care to default to all FSCK_ERROR in those cases.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Allow demoting errors to warnings via receive.fsck.warn = <key>

For example, missing emails in commit and tag objects can be demoted to
mere warnings with

	git config receive.fsck.warn = missing-email

The value is actually a comma-separated list, and there is a
corresponding receive.fsck.error setting.

In case that the same key is listed in multiple receive.fsck.* lines in
the config, the latter configuration wins.

As git receive-pack does not actually perform the checks, it hands off
the setting to index-pack or unpack-objects in the form of an optional
argument to the --strict option.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Report the ID of the error/warning

Some legacy code has objects with non-fatal fsck issues; To enable the
user to ignore those issues, let's print out the ID (e.g. when
encountering "missing-email", the user might want to call `git config
receive.fsck.warn missing-email`).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Make fsck_ident() warn-friendly

When fsck_ident() identifies a problem with the ident, it should still
advance the pointer to the next line so that fsck can continue in the
case of a mere warning.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Make fsck_commit() warn-friendly

When fsck_commit() identifies a problem with the commit, it should try
to make it possible to continue checking the commit object, in case the
user wants to demote the detected errors to mere warnings.

Note that some problems are too problematic to simply ignore. For
example, when the header lines are mixed up, we punt after encountering
an incorrect line. Therefore, demoting certain warnings to errors can
hide other problems. Example: demoting the missing-author error to
a warning would hide a problematic committer line.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Handle multiple authors in commits specially

This problem has been detected in the wild, and is the primary reason
to introduce an option to demote certain fsck errors to warnings. Let's
offer to ignore this particular problem specifically.

Technically, we could handle such repositories by setting
receive.fsck.warn = missing-committer, but that could hide missing tree
objects in the same commit because we cannot continue verifying any
commit object after encountering a missing committer line, while we can
continue in the case of multiple author lines.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Make fsck_tag() warn-friendly

When fsck_tag() identifies a problem with the commit, it should try
to make it possible to continue checking the commit object, in case the
user wants to demote the detected errors to mere warnings.

Just like fsck_commit(), there are certain problems that could hide other
issues with the same tag object. For example, if the 'type' line is not
encountered in the correct position, the 'tag' line – if there is any –
would not be handled at all.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Add a simple test for receive.fsck.*

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Disallow demoting grave fsck errors to warnings

Some kinds of errors are intrinsically unrecoverable (e.g. errors while
uncompressing objects). It does not make sense to allow demoting them to
mere warnings.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Optionally ignore specific fsck issues completely

An fsck issue in a legacy repository might be so common that one would
like not to bother the user with mentioning it at all. With this change,
that is possible by setting the respective error to "ignore".

This change "abuses" the warn=missing-email test to verify that "ignore"
is also accepted and works correctly. And while at it, it makes sure
that multiple options work, too (they are passed to unpack-objects or
index-pack as a comma-separated list via the --strict=... command-line
option).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Allow upgrading fsck warnings to errors

The 'invalid tag name' and 'missing tagger entry' warnings can now be
upgraded to errors by specifying `invalid-tag-name` and
`missing-tagger-entry` to the receive.fsck.error config setting.

Incidentally, the missing tagger warning is now really shown as a warning
(as opposed to being reported with the "error:" prefix, as it used to be
the case before this commit).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Document the new receive.fsck.* options.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Support demoting errors to warnings

We already have support in `git receive-pack` to deal with some legacy
repositories which have non-fatal issues.

Let's make `git fsck` itself useful with such repositories, too, by
allowing users to ignore known issues, or at least demote those issues
to mere warnings.

Example: `git -c fsck.ignore=missing-email fsck` would hide problems with
missing emails in author, committer and tagger lines.

In the same spirit that `git receive-pack`'s usage of the fsck machinery
differs from `git fsck`'s – some of the non-fatal warnings in `git fsck`
are fatal with `git receive-pack` when receive.fsckObjects = true, for
example – we strictly separate the fsck.* from the receive.fsck.*
settings.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Introduce `git fsck --quick`

This option avoids unpacking each and all objects, and just verifies the
connectivity. In particular with large repositories, this speeds up the
operation, at the expense of missing corrupt blobs and ignoring
unreachable objects, if any.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: git receive-pack: support excluding objects from fsck'ing

The optional new config option `receive.fsck.skiplist` specifies the path
to a file listing the names, i.e. SHA-1s, one per line, of objects that
are to be ignored by `git receive-pack` when `receive.fsckObjects = true`.

This is extremely handy in case of legacy repositories where it would
cause more pain to change incorrect objects than to live with them
(e.g. a duplicate 'author' line in an early commit object).

The intended use case is for server administrators to inspect objects
that are reported by `git push` as being too problematic to enter the
repository, and to add the objects' SHA-1 to a (preferably sorted) file
when the objects are legitimate, i.e. when it is determined that those
problematic objects should be allowed to enter the server.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>