Fix automake infinite loop

[drok]

When --dry-running autoconf projects, the default make invocation flags, along causes the dry-run process to enter an infinite loop where the Makefile is updated, by running config.status, which requires reconfiguration.

The problem

The infinite loop when dry-running autoconf projects is due to GNU Make's documented behaviour. It's not a bug, or version-specific behaviour:

1. GNU Make remakes files makefiles unconditionally (defined as files it reads rules from, eg, given a statement "include blah.inc", the file blah.inc is a "makefile"). Remaking Makefiles says "--dry-run does not prevent updating of makefiles, because an out-of-date makefile would result in the wrong output for other targets"

2. Automake creates rules for updating the Makefile when Makefile.am changes, and when configure.ac changes.

3. In recursive builds, when make spawns a sub-make process, it does not pass "--assume-old" arguments to child make via the MAKEFLAGS mechanism. This is described in Communicating Options to a Sub-make

Together, these three behaviours guarantee that an autoconf+automake project will end up in an infinite loop when "--dry-run" as the vscode "Makefile Tools" extension does by default when projects are loaded.

Solution

The solution is to change the command line options as follows (settings.json excerpt):

"makefile.dryrunSwitches": [
        "--always-make",
        "--print-directory",
        "--dry-run",
        "--assume-old=Makefile",
        "Makefile",
        "all",
        "AM_MAKEFLAGS=--assume-old=Makefile Makefile"
],

If using the GUI, each of these strings must appear on one line. Specifically, AM_MAKEFLAGS=--assume-old=Makefile Makefile is one argument. In a bash command line, this would be quoted like AM_MAKEFLAGS="--assume-old=Makefile Makefile"

Explanation

The --assume-old=Makefile means the file named 'Makefile', which is also a target, will not be considered for remake, in spite of --always-make, per Avoiding Compilation.

Since the file Makefile is also a file (ie, used to read rules from), it would be re-made in spite of "--dry-run". This behaviour is overriden by specifying Makefile as a target, in addition to all, as documented in Remaking Makefiles: "on occasion you might actually wish to prevent updating of even the makefiles. You can do this by specifying the makefiles as goals in the command line as well as specifying them as makefiles. When the makefile name is specified explicitly as a goal, the options ‘-t’ and so on do apply to them."

Since on recursive builds the --assume-old argument would not be passed to the sub-make via make's own MAKEFLAGS environment variable mechanism, the AM_MAKEFLAGS variable is used to perform override this behaviour. This works because automake projects add the value of this variable on every make invocation.

Note 1. If your project defines the AM_MAKEFLAGS variable in Makefile.am, you'll need to adjust it to include the needed --assume-old incantation. One way would be:

AM_MAKEFLAGS="--project-specific-make-flags $(DRYRUN_MAKEFLAGS)"

Then, use use DRYRUN_MAKEFLAGS="--assume-old=Makefile Makefile" in settings.json.

Note 2. Which of the settings.json files you put the "makefile.dryrunSwitches" settings matters. If you work on autoconf as well as cmake projects, you might want it in each project's settings.json. I have it set at the user level, as I work on very few cmake projects, and then I can override in at the project level.

Marked WIP because of these issues:

1. adding "all" as a target makes the assumption that "all" is the default target, which may not be true, especially in non-automake projects.
2. The workaround for recursive sub-makes only works if automake is in use. Otherwise, another way to pass --assume-old to sub-makes should be found.

The problem was reported at StackOverflow, and this is my solution to that question.

This commit changes the default makefile.dryrunSwitches flags as documented above.

Originally proposed upstream as microsoft#500