Add a print_rerun feature

Summary:
Useful for some projects that want to reuse the what-changed logic. Rough model would be:

* Run `supertd targets cell//... --output=data.json` (or with `--dry-run` to get the flags and do it yourself).
* Run `supertd btd --base=data.json --print-rerun cell//...` (with `--cells` and `--config` as per the README probably).
* Look at the output - will either be `+` for targets that were modified, `-` for ones that were deleted, or `*` if there is no benefit to doing a rerun.

Reviewed By: Acesine

Differential Revision: D56398248

fbshipit-source-id: bfbfe50443e69971fb1c7a6387c9d4d4cfa47acc

btd/src/lib.rs

@@ -141,6 +141,10 @@ pub struct Args {
     #[arg(long)]
     graph_size: bool,
 
+    /// Print out the patterns to rerun. Patterns will be prefixed with either `+` (added) or `-` (removed).
+    #[arg(long)]
+    print_rerun: bool,
+
     /// Reports all graph errors on the diff revision.
     #[arg(long)]
     write_errors_to_file: Option<PathBuf>,
@@ -199,6 +203,10 @@ pub fn main(args: Args) -> anyhow::Result<()> {
                 None => universe.to_vec(),
                 Some(x) => x.modified.map(|x| x.as_pattern()),
             };
+            if args.print_rerun {
+                print_rerun(&rerun);
+                return Ok(());
+            }
             let new = if ask_buck.is_empty() {
                 Targets::new(Vec::new())
             } else {
@@ -301,6 +309,21 @@ struct Rerun {
     deleted: HashSet<Package>,
 }
 
+/// Print out the patterns to rerun, in diff style.
+fn print_rerun(rerun: &Option<Rerun>) {
+    match rerun {
+        None => println!("* everything"),
+        Some(rerun) => {
+            for x in &rerun.deleted {
+                println!("- {}", x);
+            }
+            for x in &rerun.modified {
+                println!("+ {}", x);
+            }
+        }
+    }
+}
+
 /// Tells us which things might have changed, and therefore what
 /// we should run buck2 targets on at the diff revision to
 /// properly check if it really did change.

btd/test/test_e2e.py

@@ -110,6 +110,7 @@ def test_run(patch_name):
         out_btd1 = Path(output_dir).joinpath("btd1.json")
         out_btd2 = Path(output_dir).joinpath("btd2.json")
         out_targets = Path(output_dir).joinpath("targets.txt")
+        out_rerun = Path(output_dir).joinpath("rerun.txt")
         btd_args = [
             "--check-dangling",
             "--cells",
@@ -165,6 +166,16 @@ def test_run(patch_name):
             output=out_btd2,
             expect_fail=btd_fail,
         )
+        run(
+            btd,
+            *btd_args,
+            "--universe",
+            "root//...",
+            "--buck",
+            buck,
+            "--print-rerun",
+            output=out_rerun,
+        )
         if not btd_fail:
             # We want to make sure our rerun logic was correct
             assert read_file(out_btd1) == read_file(out_btd2)
@@ -177,6 +188,8 @@ def test_run(patch_name):
             run(buck, "build", "@" + str(out_targets))
             # Check custom properties
             check_properties(patch_name, output)
+        rerun = read_file(out_rerun)
+        check_properties_rerun(patch_name, rerun)
 
 
 def check_properties(patch, rdeps):
@@ -219,6 +232,23 @@ def check_properties(patch, rdeps):
         raise AssertionError("No properties known for: " + patch)
 
 
+def check_properties_rerun(patch, rerun):
+    if patch == "nothing":
+        assert rerun == ""
+    elif patch == "file":
+        assert rerun == ""
+    elif patch == "rename_inner":
+        assert rerun == "+ root//inner\n"
+    elif patch == "buckconfig":
+        assert rerun == "* everything\n"
+    elif patch == "cfg_modifiers":
+        assert rerun == "+ root//inner\n"
+    elif patch == "delete_inner":
+        assert rerun == "- root//inner\n"
+    else:
+        raise AssertionError("No properties known for: " + patch)
+
+
 def expect_btd_failure(patch):
     if patch == "delete_inner":
         return "Target `root//inner:baz` was deleted but is referenced by `root//:bar`"