3 files changed, 308 insertions, 2 deletions
diff --git a/Documentation/technical/api-trace2.txt b/Documentation/technical/api-trace2.txt
index de5fc25059..5817b18310 100644
--- a/Documentation/technical/api-trace2.txt
+++ b/Documentation/technical/api-trace2.txt
@@ -128,7 +128,7 @@ yields
 
 ------------
 $ cat ~/log.event
-{"event":"version","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.620713Z","file":"common-main.c","line":38,"evt":"3","exe":"2.20.1.155.g426c96fcdb"}
+{"event":"version","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.620713Z","file":"common-main.c","line":38,"evt":"4","exe":"2.20.1.155.g426c96fcdb"}
 {"event":"start","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621027Z","file":"common-main.c","line":39,"t_abs":0.001173,"argv":["git","version"]}
 {"event":"cmd_name","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621122Z","file":"git.c","line":432,"name":"version","hierarchy":"version"}
 {"event":"exit","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621236Z","file":"git.c","line":662,"t_abs":0.001227,"code":0}
@@ -344,7 +344,7 @@ only present on the "start" and "atexit" events.
 {
 	"event":"version",
 	...
-	"evt":"3",		       # EVENT format version
+	"evt":"4",		       # EVENT format version
 	"exe":"2.20.1.155.g426c96fcdb" # git version
 }
 ------------
@@ -835,6 +835,19 @@ The "value" field may be an integer or a string.
 }
 ------------
 
+`"printf"`::
+	This event logs a human-readable message with no particular formatting
+	guidelines.
++
+------------
+{
+	"event":"printf",
+	...
+	"t_abs":0.015905,      # elapsed time in seconds
+	"msg":"Hello world"    # optional
+}
+------------
+
 
 == Example Trace2 API Usage
 
diff --git a/Documentation/technical/multi-pack-index.txt b/Documentation/technical/multi-pack-index.txt
index f2221d2b44..cc063b30be 100644
--- a/Documentation/technical/multi-pack-index.txt
+++ b/Documentation/technical/multi-pack-index.txt
@@ -61,6 +61,109 @@ Design Details
 - The MIDX file format uses a chunk-based approach (similar to the
   commit-graph file) that allows optional data to be added.
 
+Incremental multi-pack indexes
+------------------------------
+
+As repositories grow in size, it becomes more expensive to write a
+multi-pack index (MIDX) that includes all packfiles. To accommodate
+this, the "incremental multi-pack indexes" feature allows for combining
+a "chain" of multi-pack indexes.
+
+Each individual component of the chain need only contain a small number
+of packfiles. Appending to the chain does not invalidate earlier parts
+of the chain, so repositories can control how much time is spent
+updating the MIDX chain by determining the number of packs in each layer
+of the MIDX chain.
+
+=== Design state
+
+At present, the incremental multi-pack indexes feature is missing two
+important components:
+
+  - The ability to rewrite earlier portions of the MIDX chain (i.e., to
+    "compact" some collection of adjacent MIDX layers into a single
+    MIDX). At present the only supported way of shrinking a MIDX chain
+    is to rewrite the entire chain from scratch without the `--split`
+    flag.
++
+There are no fundamental limitations that stand in the way of being able
+to implement this feature. It is omitted from the initial implementation
+in order to reduce the complexity, but will be added later.
+
+  - Support for reachability bitmaps. The classic single MIDX
+    implementation does support reachability bitmaps (see the section
+    titled "multi-pack-index reverse indexes" in
+    linkgit:gitformat-pack[5] for more details).
++
+As above, there are no fundamental limitations that stand in the way of
+extending the incremental MIDX format to support reachability bitmaps.
+The design below specifically takes this into account, and support for
+reachability bitmaps will be added in a future patch series. It is
+omitted from the current implementation for the same reason as above.
++
+In brief, to support reachability bitmaps with the incremental MIDX
+feature, the concept of the pseudo-pack order is extended across each
+layer of the incremental MIDX chain to form a concatenated pseudo-pack
+order. This concatenation takes place in the same order as the chain
+itself (in other words, the concatenated pseudo-pack order for a chain
+`{$H1, $H2, $H3}` would be the pseudo-pack order for `$H1`, followed by
+the pseudo-pack order for `$H2`, followed by the pseudo-pack order for
+`$H3`).
++
+The layout will then be extended so that each layer of the incremental
+MIDX chain can write a `*.bitmap`. The objects in each layer's bitmap
+are offset by the number of objects in the previous layers of the chain.
+
+=== File layout
+
+Instead of storing a single `multi-pack-index` file (with an optional
+`.rev` and `.bitmap` extension) in `$GIT_DIR/objects/pack`, incremental
+MIDXs are stored in the following layout:
+
+----
+$GIT_DIR/objects/pack/multi-pack-index.d/
+$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-chain
+$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H1.midx
+$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H2.midx
+$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H3.midx
+----
+
+The `multi-pack-index-chain` file contains a list of the incremental
+MIDX files in the chain, in order. The above example shows a chain whose
+`multi-pack-index-chain` file would contain the following lines:
+
+----
+$H1
+$H2
+$H3
+----
+
+The `multi-pack-index-$H1.midx` file contains the first layer of the
+multi-pack-index chain. The `multi-pack-index-$H2.midx` file contains
+the second layer of the chain, and so on.
+
+When both an incremental- and non-incremental MIDX are present, the
+non-incremental MIDX is always read first.
+
+=== Object positions for incremental MIDXs
+
+In the original multi-pack-index design, we refer to objects via their
+lexicographic position (by object IDs) within the repository's singular
+multi-pack-index. In the incremental multi-pack-index design, we refer
+to objects via their index into a concatenated lexicographic ordering
+among each component in the MIDX chain.
+
+If `objects_nr()` is a function that returns the number of objects in a
+given MIDX layer, then the index of an object at lexicographic position
+`i` within, say, $H3 is defined as:
+
+----
+objects_nr($H2) + objects_nr($H1) + i
+----
+
+(in the C implementation, this is often computed as `i +
+m->num_objects_in_base`).
+
 Future Work
 -----------
 
diff --git a/Documentation/technical/platform-support.txt b/Documentation/technical/platform-support.txt
new file mode 100644
index 0000000000..a227c363d7
--- /dev/null
+++ b/Documentation/technical/platform-support.txt
@@ -0,0 +1,190 @@
+Platform Support Policy
+=======================
+
+Git has a history of providing broad "support" for exotic platforms and older
+platforms, without an explicit commitment. Stakeholders of these platforms may
+want a more predictable support commitment. This is only possible when platform
+stakeholders supply Git developers with adequate tooling, so we can test for
+compatibility or develop workarounds for platform-specific quirks on our own.
+Various levels of platform-specific tooling will allow us to make more solid
+commitments around Git's compatibility with that platform.
+
+Note that this document is about maintaining existing support for a platform
+that has generally worked in the past; for adding support to a platform which
+doesn't generally work with Git, the stakeholders for that platform are expected
+to do the bulk of that work themselves. We will consider such patches if they
+don't make life harder for other supported platforms or for Git contributors.
+Some contributors may volunteer to help with the initial or continued support,
+but that's not a given. Support work which is too intrusive or difficult for the
+project to maintain may still not be accepted.
+
+Minimum Requirements
+--------------------
+
+The rest of this doc describes best practices for platforms to make themselves
+easy to support. However, before considering support at all, platforms need to
+meet the following minimum requirements:
+
+* Has C99 or C11
+
+* Uses versions of dependencies which are generally accepted as stable and
+  supportable, e.g., in line with the version used by other long-term-support
+  distributions
+
+* Has active security support (taking security releases of dependencies, etc)
+
+These requirements are a starting point, and not sufficient on their own for the
+Git community to be enthusiastic about supporting your platform. Maintainers of
+platforms which do meet these requirements can follow the steps below to make it
+more likely that Git updates will respect the platform's needs.
+
+Compatible by next release
+--------------------------
+
+To increase probability that compatibility issues introduced in a release
+will be fixed in a later release:
+
+* You should send a bug report as soon as you notice the breakage on your
+  platform. The sooner you notice, the better; watching `seen` means you can
+  notice problems before they are considered "done with review"; whereas
+  watching `master` means the stable branch could break for your platform, but
+  you have a decent chance of avoiding a tagged release breaking you. See "The
+  Policy" in link:../howto/maintain-git.txt["How to maintain Git"] for an
+  overview of which branches are used in the Git project, and how.
+
+* The bug report should include information about what platform you are using.
+
+* You should also use linkgit:git-bisect[1] and determine which commit
+  introduced the breakage.
+
+* Please include any information you have about the nature of the breakage: is
+  it a memory alignment issue? Is an underlying library missing or broken for
+  your platform? Is there some quirk about your platform which means typical
+  practices (like malloc) behave strangely?
+
+* If possible, build Git from the exact same source both for your platform and
+  for a mainstream platform, to see if the problem you noticed appears only
+  on your platform. If the problem appears in both, then it's not a
+  compatibility issue, but we of course appreciate hearing about it in a bug
+  report anyway, to benefit users of every platform. If it appears only on your
+  platform, mention clearly that it is a compatibility issue in your report.
+
+* Once we begin to fix the issue, please work closely with the contributor
+  working on it to test the proposed fix against your platform.
+
+Example: NonStop
+https://lore.kernel.org/git/01bd01da681a$b8d70a70$2a851f50$@nexbridge.com/[reports
+problems] when they're noticed.
+
+Compatible on `master` and releases
+-----------------------------------
+
+To make sure all stable builds and regular releases work for your platform the
+first time, help us avoid breaking `master` for your platform:
+
+* You should run regular tests against the `next` branch and
+  publish breakage reports to the mailing list immediately when they happen.
+
+** Ideally, these tests should run daily. They must run more often than
+   weekly, as topics generally spend at least 7 days in `next` before graduating
+   to `master`, and it takes time to put the brakes on a patch once it lands in
+   `next`.
+
+** You may want to ask to join the mailto:git-security@googlegroups.com[security
+   mailing list] in order to run tests against the fixes proposed there, too.
+
+* It may make sense to automate these; if you do, make sure they are not noisy
+  (you don't need to send a report when everything works, only when something
+  breaks; you don't need to send repeated reports for the same breakage night
+  after night).
+
+* Breakage reports should be actionable - include clear error messages that can
+  help developers who may not have access to test directly on your platform.
+
+* You should use git-bisect and determine which commit introduced the breakage;
+  if you can't do this with automation, you should do this yourself manually as
+  soon as you notice a breakage report was sent.
+
+* You should either:
+
+** Provide on-demand access to your platform to a trusted developer working to
+   fix the issue, so they can test their fix, OR
+
+** Work closely with the developer fixing the issue; the turnaround to check
+   that their proposed fix works for your platform should be fast enough that it
+   doesn't hinder the developer working on that fix. Slow testing turnarounds
+   may cause the fix to miss the next release, or the developer may lose
+   interest in working on the fix at all.
+
+Example:
+https://lore.kernel.org/git/CAHd-oW6X4cwD_yLNFONPnXXUAFPxgDoccv2SOdpeLrqmHCJB4Q@mail.gmail.com/[AIX]
+provides a build farm and runs tests against release candidates.
+
+Compatible on `next`
+--------------------
+
+To avoid reactive debugging and fixing when changes hit a release or stable, you
+can aim to ensure `next` always works for your platform. (See "The Policy" in
+link:../howto/maintain-git.txt["How to maintain Git"] for an overview of how
+`next` is used in the Git project.) To do that:
+
+* You should add a runner for your platform to the GitHub Actions or GitLab CI
+  suite.  This suite is run when any Git developer proposes a new patch, and
+  having a runner for your platform/configuration means every developer will
+  know if they break you, immediately.
+
+** If adding it to an existing CI suite is infeasible (due to architecture
+   constraints or for performance reasons), any other method which runs as
+   automatically and quickly as possible works, too. For example, a service
+   which snoops on the mailing list and automatically runs tests on new [PATCH]
+   emails, replying to the author with the results, would also be within the
+   spirit of this requirement.
+
+* If you rely on Git avoiding a specific pattern that doesn't work well with
+  your platform (like a certain malloc pattern), raise it on the mailing list.
+  We'll work case-by-case to look for a solution that doesn't unnecessarily
+  constrain other platforms to keep compatibility with yours.
+
+* If you rely on some configuration or behavior, add a test for it. Untested
+  behavior is subject to breakage at any time.
+
+** Clearly label these tests as necessary for platform compatibility. Add them
+   to an isolated compatibility-related test suite, like a new t* file or unit
+   test suite, so that they're easy to remove when compatibility is no longer
+   required.  If the specific compatibility need is gated behind an issue with
+   another project, link to documentation of that issue (like a bug or email
+   thread) to make it easier to tell when that compatibility need goes away.
+
+** Include a comment with an expiration date for these tests no more than 1 year
+   from now. You can update the expiration date if your platform still needs
+   that assurance down the road, but we need to know you still care about that
+   compatibility case and are working to make it unnecessary.
+
+Example: We run our
+https://git.kernel.org/pub/scm/git/git.git/tree/.github/workflows/main.yml[CI
+suite] on Windows, Ubuntu, Mac, and others.
+
+Getting help writing platform support patches
+---------------------------------------------
+
+In general, when sending patches to fix platform support problems, follow
+these guidelines to make sure the patch is reviewed with the appropriate level
+of urgency:
+
+* Clearly state in the commit message that you are fixing a platform breakage,
+  and for which platform.
+
+* Use the CI and test suite to ensure that the fix for your platform doesn't
+  break other platforms.
+
+* If possible, add a test ensuring this regression doesn't happen again. If
+  it's not possible to add a test, explain why in the commit message.
+
+Platform Maintainers
+--------------------
+
+If you maintain a platform, or Git for that platform, and intend to work with
+the Git project to ensure compatibility, please send a patch to add yourself to
+this list.
+
+NonStop: Randall S. Becker <rsbecker@nexbridge.com>