Debugging instructions (fixes facebookincubator#619)

Author: simonmar

glean/website/docs/devel/debug.md

@@ -0,0 +1,84 @@
+---
+id: debugging
+title: Debugging
+sidebar_label: Debugging
+---
+
+Some hints on how to debug Glean.
+
+## Logging and debug output
+
+### Glog
+
+Glean uses [glog](https://github.com/google/glog) for a lot of its
+logging both in C++ and Haskell. The Haskell API is provided by the
+`Util.Log` library in the `fb-util` package
+(`hsthrift/common/util`).
+
+Glog has various logging levels, INFO, WARN, ERROR, etc.,
+corresponding to `logInfo`, `logWarn` and so on in `Util.Log`, and
+also verbose logging via `vlog`.
+
+By default, verbose logs are suppressed but all other logs are shown,
+except in `glean shell` where we suppress `logInfo` to avoid logs
+interfering with the interactive shell. To see more logs there are
+various ways. The most foolproof is with an environment variable. To
+see verbose logs up to level 3 for example:
+
+```
+GLOG_v=3 glean-server ...
+```
+
+The shell is a bit different, because it adds an implicit
+`--minloglevel=2` argument which overrides the environment
+variable. So for the shell we must use the command-line `-v`:
+
+```
+glean -v 3 shell ...
+```
+
+Glog is sometimes a bit spammy because you'll see logs from irrelevant
+library dependencies. For C++ you can limit the logs you see to just
+certain files. For example, to turn on verbose logging at level 3 for
+just `query.cpp`:
+
+```
+GLOG_vmodule=query=3 glean-server ...
+```
+
+This useful functionality unfortunately doesn't work for logs in
+Haskell code.
+
+### Glean debug logging
+
+Glean also has its own debug logs which are enabled via environment
+variables or command-line options. There are currently two of these:
+
+* `GLEAN_DEBUG=query` or `--debug-query`: enable logging from the query compiler
+* `GLEAN_DEBUG=tc` or `--debug-tc`: enable logging from the Angle type checker
+
+## Debugging the C++ code
+
+We recommend [Address
+Sanitizer](https://github.com/google/sanitizers/wiki/addresssanitizer)
+as a first port of call for debugging anything in C++. Address
+Sanitizer will detect most kinds of memory errors and space leaks.
+
+To make it easy to enable this we've provided a `cabal` flag:
+
+```
+cabal build glean-server -fasan
+```
+
+(Note you might need to `cabal clean` first, or selectively remove
+some build files, because Cabal isn't very good at rebuilding C++ code
+when it needs to. You'll need to clean again if you turn off `asan`.)
+
+## Profiling C++
+
+`perf` is a pretty good way to identify hotspots quickly.
+
+```
+perf record -g glean-server ...
+perf report
+```

glean/website/sidebars.js

@@ -96,6 +96,7 @@ module.exports = {
            },
        ],
        'Developers': [
+           'devel/debugging',
            {
                'Implementation Notes': [
                    'implementation/db',