Do not use shallow clones for short revisions

When the `tag` of a `source-repository-stanza` is not a full hash,
we cannot do a shallow clone and fetch it separately.

Therefore, in those cases, we fall back to doing a full clone.

Fixes haskell#10605

Includes a regression test by @9999years.

Co-authored-by: Rebecca Turner <rbt@sent.as>

cabal-install/src/Distribution/Client/VCS.hs

@@ -546,51 +546,56 @@ vcsGit =
               (\e -> if isPermissionError e then removePathForcibly dotGitModulesPath else throw e)
           else removeDirectoryRecursive dotGitModulesPath
 
-      -- If we want a particular branch or tag, fetch it.
-      ref <- case srpBranch `mplus` srpTag of
-        Nothing -> pure "HEAD"
-        Just ref -> do
-          -- /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\
-          -- /!\            MULTIPLE HOURS HAVE BEEN LOST HERE!!             /!\
-          -- /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\
-          --
-          -- If you run `git fetch origin MY_TAG`, then the tag _will_ be
-          -- fetched, but no local ref (e.g. `refs/tags/MY_TAG`) will be
-          -- created.
-          --
-          -- This means that doing `git fetch origin MY_TAG && git reset --hard
-          -- MY_TAG` will fail with a message like `unknown revision MY_TAG`.
-          --
-          -- There are two ways around this:
-          --
-          -- 1. Provide a refmap explicitly:
-          --
-          --        git fetch --refmap="+refs/tags/*:refs/tags/*" origin MYTAG
-          --
-          --    This tells Git to create local tags matching remote tags. It's
-          --    not in the default refmap so you need to set it explicitly.
-          --    (You can also set it with `git config set --local
-          --    remote.origin.fetch ...`.)
-          --
-          -- 2. Use `FETCH_HEAD` directly: Git writes a `FETCH_HEAD` ref
-          --    containing the commit that was just fetched. This feels a bit
-          --    nasty but seems to work reliably, even if nothing was fetched.
-          --    (That is, deleting `FETCH_HEAD` and re-running a `git fetch`
-          --    command will succesfully recreate the `FETCH_HEAD` ref.)
-          --
-          -- Option 2 is what Cabal has done historically, and we're keeping it
-          -- for now. Option 1 is possible but seems to have little benefit.
-          git localDir ("fetch" : verboseArg ++ ["origin", ref])
-          pure "FETCH_HEAD"
-
-      -- Then, reset to the appropriate ref.
-      git localDir $
-        "reset"
-          : verboseArg
-          ++ [ "--hard"
-             , ref
-             , "--"
-             ]
+      -- `doShallow` controls whether we use a shallow clone.
+      -- If the clone is shallow, make sure to fetch specified revisions before
+      -- using them.
+      when doShallow $ do
+
+        -- If we want a particular branch or tag, fetch it.
+        ref <- case srpBranch `mplus` srpTag of
+          Nothing -> pure "HEAD"
+          Just ref -> do
+            -- /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\
+            -- /!\            MULTIPLE HOURS HAVE BEEN LOST HERE!!             /!\
+            -- /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\ /!\
+            --
+            -- If you run `git fetch origin MY_TAG`, then the tag _will_ be
+            -- fetched, but no local ref (e.g. `refs/tags/MY_TAG`) will be
+            -- created.
+            --
+            -- This means that doing `git fetch origin MY_TAG && git reset --hard
+            -- MY_TAG` will fail with a message like `unknown revision MY_TAG`.
+            --
+            -- There are two ways around this:
+            --
+            -- 1. Provide a refmap explicitly:
+            --
+            --        git fetch --refmap="+refs/tags/*:refs/tags/*" origin MYTAG
+            --
+            --    This tells Git to create local tags matching remote tags. It's
+            --    not in the default refmap so you need to set it explicitly.
+            --    (You can also set it with `git config set --local
+            --    remote.origin.fetch ...`.)
+            --
+            -- 2. Use `FETCH_HEAD` directly: Git writes a `FETCH_HEAD` ref
+            --    containing the commit that was just fetched. This feels a bit
+            --    nasty but seems to work reliably, even if nothing was fetched.
+            --    (That is, deleting `FETCH_HEAD` and re-running a `git fetch`
+            --    command will succesfully recreate the `FETCH_HEAD` ref.)
+            --
+            -- Option 2 is what Cabal has done historically, and we're keeping it
+            -- for now. Option 1 is possible but seems to have little benefit.
+            git localDir ("fetch" : verboseArg ++ ["origin", ref])
+            pure "FETCH_HEAD"
+
+        -- Then, reset to the appropriate ref.
+        git localDir $
+          "reset"
+            : verboseArg
+            ++ [ "--hard"
+               , ref
+               , "--"
+               ]
 
       -- We need to check if `.gitmodules` exists _after_ the `git reset` call.
       gitModulesExists <- doesFileExist gitModulesPath
@@ -608,8 +613,31 @@ vcsGit =
               { progInvokeCwd = Just cwd
               }
 
+        -- Beware: if the user supplied revision for the source repository
+        -- package is /not/ a full hash, then we cannot fetch it, which means
+        -- we cannot do a shallow clone (--depth=1).
+        -- See #10605.
+        doShallow
+          | Nothing <- srpTag
+          -- No tag, OK for shallow
+          = True
+
+          -- full hashes are 40 characters
+          | Just tg <- srpTag
+          , length tg >= 40
+          = True
+
+          | otherwise
+          = False
+
+        depthIs1
+          | doShallow = ["--depth=1"]
+          | otherwise = []
+
         cloneArgs =
-          ["clone", "--depth=1", "--no-checkout", loc, localDir]
+          ["clone" ]
+            ++ depthIs1
+            ++ [ "--no-checkout", loc, localDir]
             ++ case peer of
               Nothing -> []
               Just peerLocalDir -> ["--reference", peerLocalDir]

cabal-testsuite/PackageTests/SourceRepositoryPackageShallow/cabal.out

@@ -0,0 +1,10 @@
+# cabal v2-build
+Configuration is affected by the following files:
+- cabal.project
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following will be built:
+ - puppy-1.0 (lib) (first run)
+Configuring library for puppy-1.0...
+Preprocessing library for puppy-1.0...
+Building library for puppy-1.0...

cabal-testsuite/PackageTests/SourceRepositoryPackageShallow/cabal.project

@@ -0,0 +1,8 @@
+packages: .
+
+-- Regression for https://github.com/haskell/cabal/issues/10605
+-- This is `my-lib` 0.9.
+source-repository-package
+  type: git
+  location: https://github.com/9999years/cabal-testsuite-my-lib.git
+  tag: 9a0af0aa

cabal-testsuite/PackageTests/SourceRepositoryPackageShallow/cabal.test.hs

@@ -0,0 +1,4 @@
+import Test.Cabal.Prelude
+
+main = cabalTest $ do
+  cabal "v2-build" []

cabal-testsuite/PackageTests/SourceRepositoryPackageShallow/puppy.cabal

@@ -0,0 +1,9 @@
+cabal-version:      3.0
+name:               puppy
+version:            1.0
+
+library
+    default-language: Haskell2010
+    hs-source-dirs:   src
+    build-depends:    base
+    exposed-modules:  Puppy

cabal-testsuite/PackageTests/SourceRepositoryPackageShallow/src/Puppy.hs

@@ -0,0 +1 @@
+module Puppy () where