Refusing an output under a non-directory depends on rule order, because putup learns a path's type only when a rule names it #495

Symptom

Whether putup refuses an output under a non-directory depends on rule order. PR #494 refuses sub/x.o when a rule has already named the file sub. Put the output rule first, and the command runs and fails in the shell instead:

: |> echo hi > %o |> sub/x.o
: sub |> cat %f > %o |> a.txt

sub is a regular file in the source tree.

$ putup -B .
/bin/sh: 1: cannot create sub/x.o: Directory nonexistent
[.] Build completed: 1 commands (1 failed) in 1ms

The reverse case has the same cause. A path that already holds a Directory node can be claimed as an output, and the command then fails in the shell:

: |> echo b > %o |> gen/x
: |> echo a > %o |> gen

/bin/sh: 1: cannot create gen: Is a directory

Both were run against build/putup from the #494 branch.

Upstream

tup parse refuses both at parse time:

tup error: Unable to output to a different directory because 'sub' is a normal file
tup error: Failed to find directory ID for dir 'sub/x.o' relative to '.'
tup error: Error parsing Tupfile line 1

tup error: Attempting to insert 'gen' as a generated node when it already exists as a different type (generated directory). ...
tup error: Error parsing Tupfile line 2

The first refusal comes from find_dir_tupid_dt_pg; tup knows sub is a file from its filesystem scan, so rule order does not matter. The second comes from validate_output.

Cause

putup learns a source path's type only when a rule names it:

• ensure_file_node (src/graph/dag.cpp) creates each missing intermediate component as NodeType::Directory without consulting the disk. So the output rule mints a Directory node for sub. The later input sub then finds that node through path_to_node and never retypes it.
• ensure_file_node(…, NodeType::Generated) promotes an existing Ghost or File node to Generated, but returns an existing Directory node unchanged.

Fix direction

Both cases are type checks at the point where ensure_file_node binds an existing or new node:

• An intermediate component that exists on disk as a non-directory should be refused.
• A Generated request that lands on a Directory node should be refused, following validate_output.

REQ-OUTPUT-UNDER-DIRECTORY (spec/requirements/output-paths.ears.md, added in #494) records the first half of this gap in its reference.