Handle directory-file and file-directory comparisons in the diff

[TanmayPatil105]

GNU diff treats diff DIRECTORY FILE as diff DIRECTORY/FILE FILE

$ echo a > a
$ mkdir d
$  echo da > d/a
$ diff -u d a
--- d/a	2024-04-20 22:16:11.145369745 +0530
+++ a	2024-04-20 22:15:57.785813179 +0530
@@ -1 +1 @@
-da
+a
$ diff -u a d
--- a	2024-04-20 22:15:57.785813179 +0530
+++ d/a	2024-04-20 22:16:11.145369745 +0530
@@ -1 +1 @@
-a
+da
$ cargo run -- -u d a
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running `target/debug/diffutils -u d a`
--- d/a	2024-04-20 22:16:11.145369745 +0530
+++ a	2024-04-20 22:15:57.785813179 +0530
@@ -1 +1 @@
-da
+a
$ cargo run -- -u a d
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running `target/debug/diffutils -u a d`
--- a	2024-04-20 22:15:57.785813179 +0530
+++ d/a	2024-04-20 22:16:11.145369745 +0530
@@ -1 +1 @@
-a
+da

[TanmayPatil105]

Well, this will handle diff -u d a but not diff -u - a < d.

$ diff -u - a < d
--- -/a	2024-04-20 22:16:11.145369745 +0530
+++ a	2024-04-20 22:15:57.785813179 +0530
@@ -1 +1 @@
-da
+a
$ cargo run -- -u - a < d
    Finished dev [unoptimized + debuginfo] target(s) in 0.10s
     Running `target/debug/diffutils -u - a`
Failed to read from-file: Is a directory (os error 21)

GNU diff (3.9+) implementes stdin directory input using openat
See: https://git.savannah.gnu.org/cgit/diffutils.git/tree/src/diff.c#n1541

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.82%. Comparing base (8de0ca6) to head (8c6a648).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   81.78%   81.82%   +0.03%     
==========================================
  Files          10       10              
  Lines        2877     2927      +50     
  Branches      740      747       +7     
==========================================
+ Hits         2353     2395      +42     
- Misses        503      509       +6     
- Partials       21       23       +2     

Flag	Coverage Δ
macos_latest	82.70% <100.00%> (+0.59%)	⬆️
ubuntu_latest	75.19% <76.66%> (+0.44%)	⬆️
windows_latest	15.24% <20.00%> (?)

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[sylvestre]

sorry but your test fails on windows with:

---- read_from_directory stdout ----
---- read_from_directory stderr ----
thread 'main' panicked at tests\integration.rs:257:5:
assertion `left == right` failed
  left: "--- target/integration/d\\a\tTIMESTAMP\n+++ target/integration/a\tTIMESTAMP\n@@ -1 +1 @@\n-da\n+a\n"
 right: "--- target/integration/d/a\tTIMESTAMP\n+++ target/integration/a\tTIMESTAMP\n@@ -1 +1 @@\n-da\n+a\n"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[oSoMoN]

Well, this will handle diff -u d a but not diff -u - a < d.

That would be nice to have, but it can be implemented in a followup PR.