run-tests: mechanism to report exceptions during test execution Sometimes when running tests you introduce a ton of exceptions. The most extreme example of this is running Mercurial with Python 3, which currently spews thousands of exceptions when running the test harness. This commit adds an opt-in feature to run-tests.py to aggregate exceptions encountered by `hg` when running tests. When --exceptions is used, the test harness enables the "logexceptions" extension in the test environment. This extension wraps the Mercurial function to handle exceptions and writes information about the exception to a random filename in a directory defined by the test harness via an environment variable. At the end of the test harness, these files are parsed, aggregated, and a list of all unique Mercurial frames triggering exceptions is printed in order of frequency. This feature is intended to aid Python 3 development. I've only really tested it on Python 3. There is no shortage of improvements that could be made. e.g. we could write a separate file containing the exception report - maybe even an HTML report. We also don't capture which tests demonstrate the exceptions, so there's no turnkey way to test whether a code change made an exception disappear. Perfect is the enemy of good. I think the current patch is useful enough to land. Whoever uses it can send patches to imprve its usefulness. Differential Revision: https://phab.mercurial-scm.org/D1477

The censor system allows retroactively removing content from
files. Actually censoring a node requires using the censor extension,
but the functionality for handling censored nodes is partially in core.

Censored nodes in a filelog have the flag ``REVIDX_ISCENSORED`` set,
and the contents of the censored node are replaced with a censor
tombstone. For historical reasons, the tombstone is packed in the
filelog metadata field ``censored``. This allows censored nodes to be
(mostly) safely transmitted through old formats like changegroup
versions 1 and 2. When using changegroup formats older than 3, the
receiver is required to re-add the ``REVIDX_ISCENSORED`` flag when
storing the revision. This depends on the ``censored`` metadata key
never being used for anything other than censoring revisions, which is
true as of January 2017. Note that the revlog flag is the
authoritative marker of a censored node: the tombstone should only be
consulted when looking for a reason a node was censored or when revlog
flags are unavailable as mentioned above.

The tombstone data is a free-form string. It's expected that users of
censor will want to record the reason for censoring a node in the
tombstone. Censored nodes must be able to fit in the size of the
content being censored.