commit 631e467325a191a92d2e9454dc9b0fe6e3f21f80
author lberki <lberki@users.noreply.github.com> Tue Jul 14 20:41:18 2020 +0200
committer GitHub <noreply@github.com> Tue Jul 14 11:41:18 2020 -0700
tree c66c4d5b4859272d785064e8fa7e30e3f734e746
parent 06a798c4db92ec3a3a6b3a4b52026f002002affa

Implement Makefile inclusion and variable assignment tracing (#203)

Introduce value assignment tracing and include tracing.

The `--dump_variable_assignment_trace <path>` command line option dumps the following data for every variable reference:
  * The stack trace of the variable reference
  * The stack trace where the value was assigned

The `--dump_include_json <path>`  option dumps the Makefile inclusion graph.

If `<path>` is `-`, the output is dumped to stderr.

If tracing is disabled, its CPU/RAM overhead is negligible according to rudimentary measurements. When enabled, it incurs a ~2x increase in memory use and a ~15x increase in execution time (dominated by I/O, since the output is quite wordy; without I/O it's ~10%)

The variable assignment trace correctly reports:
  * Simply and recursively expanded variables
  * Regular and rule-specific variables
  * Automatic variables (although the call stack where their value is set is currently not indicated; this is fixable)

Some example `jq` commands for the include graph:

List of Makefiles including a specific one:
```
jq '.nodes[] | select((.includes | index("$MAKEFILE") != null)) | .file'
```
List of Makefiles included by a Makefile:

```
jq '.nodes[] | select(.file == "$MAKEFILE")'
```
List of all seen Makefiles:

```
jq '.nodes[].file'
```