Add a "commands" tool

This tool performs a post-order traversal of the build graph, starting
from a list of targets specified on the command line, and for each
build statement encountered, prints the evaluated command line.
Use cases include:

- Generating input for a tool which needs to know the full command line
  for every command invoked during a build.  Many static analysis
  and indexing tools require this information.

- Generating a build script which does not depend on Ninja.
  For example, such a script could be used by Ninja to bootstrap
  itself.

2 files changed, 42 insertions, 6 deletions

diff --git a/doc/manual.asciidoc b/doc/manual.asciidoc
index d59f471..bc15a1f 100644
--- a/doc/manual.asciidoc
+++ b/doc/manual.asciidoc
@@ -382,6 +382,10 @@ than the _depth_ mode.  It returns non-zero if an error occurs.
 one.  It can be used to know which rule name to pass to
 +ninja -t targets rule _name_+.
 
+`commands`:: given a list of targets, print a list of commands which, if
+executed in order, may be used to rebuild those targets, assuming that all
+output files are out of date.
+
 `clean`:: remove built files.  If used like this +ninja -t clean+ it removes
 all the built files, except for those created by the generator.  If used
 like this +ninja -t clean -g+ it also removes built files created by the
diff --git a/src/ninja.cc b/src/ninja.cc
index a058e95..47693d1 100644
--- a/src/ninja.cc
+++ b/src/ninja.cc
@@ -65,12 +65,13 @@ void Usage(const BuildConfig& config) {
 "  -t TOOL  run a subtool.\n"
 "           terminates toplevel options; further flags are passed to the tool.\n"
 "           tools are:\n"
-"             browse  browse dependency graph in a web browser\n"
-"             graph   output graphviz dot file for targets\n"
-"             query   show inputs/outputs for a path\n"
-"             targets list targets by their rule or depth in the DAG\n"
-"             rules   list all rules\n"
-"             clean   clean built files\n",
+"             browse   browse dependency graph in a web browser\n"
+"             graph    output graphviz dot file for targets\n"
+"             query    show inputs/outputs for a path\n"
+"             targets  list targets by their rule or depth in the DAG\n"
+"             rules    list all rules\n"
+"             commands list all commands required to rebuild given targets\n"
+"             clean    clean built files\n",
           config.parallelism);
 }
 
@@ -357,6 +358,35 @@ int CmdRules(State* state, int argc, char* argv[]) {
   return 0;
 }
 
+void PrintCommands(Edge* edge, set<Edge*>* seen) {
+  if (!edge)
+    return;
+  if (!seen->insert(edge).second)
+    return;
+
+  for (vector<Node*>::iterator in = edge->inputs_.begin();
+       in != edge->inputs_.end(); ++in)
+    PrintCommands((*in)->in_edge_, seen);
+
+  if (!edge->is_phony())
+    puts(edge->EvaluateCommand().c_str());
+}
+
+int CmdCommands(State* state, int argc, char* argv[]) {
+  vector<Node*> nodes;
+  string err;
+  if (!CollectTargetsFromArgs(state, argc, argv, &nodes, &err)) {
+    Error("%s", err.c_str());
+    return 1;
+  }
+
+  set<Edge*> seen;
+  for (vector<Node*>::iterator in = nodes.begin(); in != nodes.end(); ++in)
+    PrintCommands((*in)->in_edge_, &seen);
+
+  return 0;
+}
+
 int CmdClean(State* state, int argc, char* argv[], const BuildConfig& config) {
   bool generator = false;
   bool clean_rules = false;
@@ -489,6 +519,8 @@ reload:
       return CmdTargets(&state, argc, argv);
     if (tool == "rules")
       return CmdRules(&state, argc, argv);
+    if (tool == "commands")
+      return CmdCommands(&state, argc, argv);
     // The clean tool uses getopt, and expects argv[0] to contain the name of
     // the tool, i.e. "clean".
     if (tool == "clean")