How to inspect the sandbox for cram tests

[Blaisorblade]

Cram tests run into a temporary sandbox folder that gets deleted — is there a way to prevent deletion, to allow inspecting the sandbox or even modifying it (when debugging the test)?

In my case, my cram tests generates some file foo.v, and then compiles it with Coq. I'm getting unexpected errors so I'd like to inspect foo.v — the only way I found is to add cat foo.v to my cram test, use promotion to update run.t, then copy-paste the output out of the .t. But this seems hacky, and would get worse if I had multiple generated files, or some of them were binary (and yes, something like base64 encoding could make the hack applicable there too).

Or I can extract the commands from the run.t file and run them outside — again, a hassle if there are many commands.

All this gets worse if the problem isn't repeatable — but I wouldn't restrict a solution to failing tests: I might want to inspect the sandbox even when the test succeeds.

EDIT: I already tried --sandbox=none — but I can't find outputs, and if run.t needs changes, the diff shows that the test still runs under _build/.sandbox. Example:

dune b --sandbox=none @test_float_lit
Entering directory '/Users/pgiarrusso1/git-bedrock/bhv'
 llvm-config fmdeps/cpp2v-core/rocq-bluerock-cpp2v/build/llvm-lib-path.txt
         sed fmdeps/cpp2v-core/rocq-bluerock-cpp2v/build/llvm-lib-path.txt
File "fmdeps/cpp2v-core/rocq-bluerock-brick/tests/cpp2v/test_float_lit.t/run.t", line 1, characters 0-0:
diff --git a/_build/.sandbox/17a92c64d74332be4f62d9b0d978432b/default/fmdeps/cpp2v-core/rocq-bluerock-brick/tests/cpp2v/test_float_lit.t/run.t b/_build/.sandbox/17a92c64d74332be4f62d9b0d978432b/default/fmdeps/cpp2v-core/rocq-bluerock-brick/tests/cpp2v/test_float_lit.t/run.t.corrected
index 7aa49eecb5..0ad369b408 100644
--- a/_build/.sandbox/17a92c64d74332be4f62d9b0d978432b/default/fmdeps/cpp2v-core/rocq-bluerock-brick/tests/cpp2v/test_float_lit.t/run.t
+++ b/_build/.sandbox/17a92c64d74332be4f62d9b0d978432b/default/fmdeps/cpp2v-core/rocq-bluerock-brick/tests/cpp2v/test_float_lit.t/run.t.corrected
[...]

[rgrinberg]

@Alizter I think you've added a setting to keep a sandbox around if a rule fails? Shall we bring that back perhaps. I think we can just go ahead with something like that except instead of keeping the sandbox spot, we can move it into some safe space for such broken sandboxes.