unikernel/duniverse/dune_/test/blackbox-tests/test-cases/dune-cache/dedup.t

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+Test deduplication of build artifacts when using Dune cache with hard links.
+
+  $ export DUNE_CACHE=enabled
+  $ export DUNE_CACHE_ROOT=$PWD/.cache
+
+  $ cat > dune-project <<EOF
+  > (lang dune 2.1)
+  > EOF
+  $ cat > dune <<EOF
+  > (rule
+  >   (deps source)
+  >   (targets target)
+  >   (action (copy source target)))
+  > EOF
+  $ cat > source <<EOF
+  > \_o< COIN
+  > EOF
+
+Here we build [target], which is a copy of [source]. After the build, the same
+file will appear in the build directory twice: (i) as [_build/default/source],
+because all sources are first copied to the build directory, and (ii) as
+[_build/default/target], as the build result. Furthermore, the same file will
+also be stored to the build cache, somewhere in the [files/v4] directory. Dune
+cache will recognise that all three files are identical and will use hard links
+to share them on disk, hence the hard link counts of 3.
+
+  $ dune build target
+  $ cat _build/default/target
+  \_o< COIN
+  $ dune_cmd stat hardlinks _build/default/source
+  3
+  $ dune_cmd stat hardlinks _build/default/target
+  3
+
+Demonstrate that sharing of build artifacts can be abused to corrupt the cache.
+
+  $ chmod 777 _build/default/target
+  $ echo "\_o< MEOW" > _build/default/target
+  $ rm -rf _build
+  $ dune build target
+  $ cat _build/default/target
+  \_o< MEOW
+
+Test that by using the [copy] mode we can disable the sharing.
+
+  $ rm -rf $DUNE_CACHE_ROOT
+  $ rm -rf _build
+  $ dune build target --cache-storage-mode=copy
+  $ cat _build/default/target
+  \_o< COIN
+  $ dune_cmd stat hardlinks _build/default/source
+  1
+  $ dune_cmd stat hardlinks _build/default/target
+  1
+
+In the [copy] mode, we can't corrupt the cache so easily.
+
+  $ chmod 777 _build/default/target
+  $ echo "\_o< MEOW" > _build/default/target
+  $ rm -rf _build
+  $ dune build target
+  $ cat _build/default/target
+  \_o< COIN