Non-matching structured hashes with head.hackage

After successfully building with head.hackage, I'm seeing build failures because the index is locked. I don't yet have a reproduction but have seen this yesterday and today.

```
$ cabal build all --enable-tests --enable-benchmarks --dry-run --minimize-conflict-set
Warning: this is a debug build of cabal-install with assertions enabled.
Warning: Parsing the index cache failed (Data.Binary.Get.runGet at position
16: Non-matching structured hashes: f46da61e7afa58a5e8fd1d2b6fb79899;
expected: d81bdd513f41b5d7ee4cd28455adadbe). Trying to regenerate the index
cache...
Warning: Parsing the index cache failed (Data.Binary.Get.runGet at position
16: Non-matching structured hashes: f46da61e7afa58a5e8fd1d2b6fb79899;
expected: d81bdd513f41b5d7ee4cd28455adadbe). Trying to regenerate the index
cache...
~/.cabal/packages/head.hackage.ghc.haskell.org/01-index.tar: withFile: resource busy (file is locked)
```

The workaround I'm using is to delete `~/.cabal/packages/head.hackage.ghc.haskell.org` followed up by `cabal update`. Could this be a problem if we start using head.hackage in CI?

## Set up of `head.hackage`

```
$ curl https://ghc.gitlab.haskell.org/head.hackage/cabal.project >> cabal.project.local
$ cabal update
```

I followed the above [head.hackage instructions](https://ghc.gitlab.haskell.org/head.hackage/) with master. The default `cabal.project` builds but not at first;

```
$ cabal --version
cabal-install version 3.11.0.0
compiled using version 3.11.0.0 of the Cabal library

$ git rev-parse HEAD
46e822152c9b3f38044870115d7a198b3a509d66

$ cabal build all --enable-tests --enable-benchmarks --dry-run
Warning: this is a debug build of cabal-install with assertions enabled.
Resolving dependencies...
Error: [Cabal-7107]
Could not resolve dependencies:
[__0] next goal: Cabal (user goal)
[__0] rejecting: Cabal-3.11.0.0
(cabal.project.local requires ==2.4.1.0 || ==3.0.2.0 || ==3.2.1.0)
[__0] rejecting: Cabal-3.10.2.1, Cabal-3.10.2.0/installed-fd40, Cabal-3.10.2.0, Cabal-3.10.1.0, Cabal-3.8.1.0, Cabal-3.6.3.0, Cabal-3.6.2.0, Cabal-3.6.1.0, Cabal-3.6.0.0, Cabal-3.4.1.0, Cabal-3.4.0.0, Cabal-3.2.1.0, Cabal-3.2.0.0, Cabal-3.0.2.0, Cabal-3.0.1.0, Cabal-3.0.0.0, Cabal-2.4.1.0, Cabal-2.4.0.1, Cabal-2.4.0.0, Cabal-2.2.0.1, Cabal-2.2.0.0, Cabal-2.0.1.1, Cabal-2.0.1.0, Cabal-2.0.0.2, Cabal-1.24.2.0, Cabal-1.24.0.0, Cabal-1.22.8.0, Cabal-1.22.7.0, Cabal-1.22.6.0, Cabal-1.22.5.0, Cabal-1.22.4.0, Cabal-1.22.3.0, Cabal-1.22.2.0, Cabal-1.22.1.1, Cabal-1.22.1.0, Cabal-1.22.0.0, Cabal-1.20.0.4, Cabal-1.20.0.3, Cabal-1.20.0.2, Cabal-1.20.0.1, Cabal-1.20.0.0, Cabal-1.18.1.7, Cabal-1.18.1.6, Cabal-1.18.1.5, Cabal-1.18.1.4, Cabal-1.18.1.3, Cabal-1.18.1.2, Cabal-1.18.1.1, Cabal-1.18.1, Cabal-1.18.0, Cabal-1.16.0.3, Cabal-1.16.0.2, Cabal-1.16.0.1, Cabal-1.16.0, Cabal-1.14.0, Cabal-1.12.0, Cabal-1.10.2.0, Cabal-1.10.1.0, Cabal-1.10.0.0, Cabal-1.8.0.6, Cabal-1.8.0.4, Cabal-1.8.0.2, Cabal-1.6.0.3, Cabal-1.6.0.2, Cabal-1.6.0.1, Cabal-1.4.0.2, Cabal-1.4.0.1, Cabal-1.4.0.0, Cabal-1.2.4.0, Cabal-1.2.3.0, Cabal-1.2.2.0, Cabal-1.2.1, Cabal-1.1.6, Cabal-1.24.1.0 (constraint from user target requires ==3.11.0.0)
[__0] fail (backjumping, conflict set: Cabal)
After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: Cabal
```

I can fix this by commenting out the constraint conflict on `Cabal` in `cabal.project.local`;

```diff
$ git diff
diff --git a/cabal.project.local b/cabal.project.local
index 576224433..9b6fb23cb 100644
--- a/cabal.project.local
+++ b/cabal.project.local
@@ -31,7 +31,7 @@ constraints:
template-haskell installed

constraints:
- Cabal ==2.4.1.0 || ==3.0.2.0 || ==3.2.1.0,
+ -- Cabal ==2.4.1.0 || ==3.0.2.0 || ==3.2.1.0,
Cabal-syntax ==3.8.1.0,
FPretty ==1.1,
JuicyPixels ==3.3.8,
```

Try again and there's another conflict for `Cabal-syntax` that I can fix the same way;

```
$ cabal build all --enable-tests --enable-benchmarks --dry-run
Warning: this is a debug build of cabal-install with assertions enabled.
Resolving dependencies...
Error: [Cabal-7107]
Could not resolve dependencies:
[__0] next goal: Cabal-syntax (user goal)
[__0] rejecting: Cabal-syntax-3.11.0.0
(constraint from cabal.project.local requires ==3.8.1.0)
[__0] rejecting: Cabal-syntax-3.10.2.0/installed-8eb4, Cabal-syntax-3.10.2.0, Cabal-syntax-3.10.1.0, Cabal-syntax-3.8.1.0, Cabal-syntax-3.6.0.0 (constraint from user target requires ==3.11.0.0)
[__0] fail (backjumping, conflict set: Cabal-syntax)
After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: Cabal-syntax
```

```diff
$ git diff
diff --git a/cabal.project.local b/cabal.project.local
index 576224433..c92471375 100644
--- a/cabal.project.local
+++ b/cabal.project.local
@@ -31,8 +31,8 @@ constraints:
template-haskell installed

constraints:
- Cabal ==2.4.1.0 || ==3.0.2.0 || ==3.2.1.0,
- Cabal-syntax ==3.8.1.0,
+ -- Cabal ==2.4.1.0 || ==3.0.2.0 || ==3.2.1.0,
+ -- Cabal-syntax ==3.8.1.0,
FPretty ==1.1,
JuicyPixels ==3.3.8,
ansi-pretty ==0.1.2.2,
```

Now `cabal build ...` works.

_Originally posted by @philderbeast in https://github.com/haskell/cabal/issues/9808#issuecomment-2009648095_

Start by reproducing the head.hackage setup from the linked instructions, then run the shown `cabal build ... --dry-run --minimize-conflict-set` command and inspect the affected `~/.cabal/packages/head.hackage.ghc.haskell.org` index cache. The report currently has no reproduction; done should include a reliable reproduction and verified behavior for the locked index and structured-hash failure.

Written by the indexing model from the issue text.