Support Ninja implicit outputs

Add .KATI_IMPLICIT_OUTPUTS target-specific variable to add extra
implicit outputs to the build line in the ninja file. This doesn't
actually affect the node graph within Kati, but combined with
--gen_all_targets should be usable.

This allows us to properly define the build graph in ninja when one rule
creates multiple files. There are a number of ways that people attempt
to do this in make, all of which have problems:

a b:
	touch a b

Define multiple outputs with the same rule. This looks correct to most
people, but actually runs the rule multiple times -- once for every
output file. Most of the time this works, but runs into very strange
concurrency issues when the two rules happen to run at the same time.

a:
	touch a b

Only define one of the files in the graph. Then users need to depend on
a file that they don't use, and the unlisted file won't be noticed if
it's updated separately (or even removed).

b: a
a:
	touch a b

Define dependencies between the multiple files. This doesn't actually
know how to update `b` if `b` is older than `a`, or doesn't exist.

b: a
	touch b
a:
	touch a b

A variant of the previous approach, which can "fix" the timestamp if the
original rule happened to create `b` before `a`. This can be even more
dangerous, since it may create / update `b` improperly.

But since ninja supports a graph where there can be multiple outputs
from a single edge, with this patch we can write:

a: .KATI_IMPLICIT_OUTPUTS := b
a:
	touch a b

which translates to:

rule rule1
  command = touch a b
build a | b: rule1

This ninja will allow users to depend on either `a` or `b`, and the rule
will be run (once) as appropriate.

Change-Id: Ieb9ced7eb4d212b0aa4de5ceb1deb9ba48ada132

1 files changed, 1 insertions, 0 deletions

diff --git a/dep.h b/dep.h
index 5e879e3..b6f25e1 100644
--- a/dep.h
+++ b/dep.h
@@ -45,6 +45,7 @@ struct DepNode {
   vector<Symbol> actual_order_only_inputs;
   Vars* rule_vars;
   Var* depfile_var;
+  Var* implicit_outputs_var;
   Var* ninja_pool_var;
   Symbol output_pattern;
   Loc loc;