Install s/ccache only when needed, note in README

It is better to not preinstall sccache/ccache in the chroot as mentioned
in the README.

README.md

@@ -63,6 +63,17 @@ GiB somehow).
 the build will fail if the limit is reached. If set to false, then the build
 will continue even if the limit is reached.
 
+## Soft-lock if `sccache` is preinstalled in chroot
+
+Apparently, some packages automatically use ccache/sccache if it is installed in
+the chroot, and in some cases, causes a soft-lock during a build. It is
+recommended to not have ccache/sccache preinstalled in the chroot and to just
+let the aur-helper-script install it when necessary.
+
+For example, when building `tenacity-git` with sccache preinstalled, the build
+will hang after the final build step. Apparently, killing the running `sccache`
+process stops the soft-lock in this case.
+
 # Setting up the AUR Helper
 
 The AUR Helper requires several things:
@@ -86,7 +97,9 @@ The `python-toml` package is required for the Python script to run.
 
 Use `/usr/bin/mkarchroot` to create your CHROOT in a directory.
 
-    mkarchroot $HOME/mychroot/root base base-devel ccache sccache cmake ninja
+    mkarchroot $HOME/mychroot/root base base-devel cmake ninja
+
+As noted earlier, it is better to NOT preinstall `ccache` and `sccache`.
 
 You must refer to the CHROOT as `$HOME/mychroot` if you used the same name as in
 the previous example.

update.py

@@ -742,6 +742,13 @@ def get_pkgbuild_version(
                     f'{os.environ["HOME"]}/.cargo/git:/build/.cargo/git',
                 )
             for dep in pkg_state[pkg]["other_deps"]:
+                prefetch_result = prefetch_dependency(dep, other_state)
+                if prefetch_result != "fetched":
+                    log_print(
+                        'ERROR: Failed to prefetch dep "{}"'.format(dep),
+                        other_state=other_state,
+                    )
+                    return False, None, None, None
                 dep_fullpath = get_latest_pkg(dep, "/var/cache/pacman/pkg")
                 if not dep_fullpath:
                     log_print(
@@ -1374,7 +1381,20 @@ def update_pkg_list(
             "--holdver",
         ]
         failure = False
+        if "ccache_dir" in pkg_state[pkg]:
+            pkg_state[pkg]["other_deps"].append("ccache")
+        elif "sccache_dir" in pkg_state[pkg]:
+            pkg_state[pkg]["other_deps"].append("sccache")
         for dep in pkg_state[pkg]["other_deps"]:
+            prefetch_result = prefetch_dependency(dep, other_state)
+            if prefetch_result != "fetched":
+                log_print(
+                    'ERROR: Failed to prefetch dep "{}"'.format(dep),
+                    other_state=other_state,
+                )
+                failure = True
+                pkg_state[pkg]["build_status"] = "get_dep_fail"
+                break
             dep_fullpath = get_latest_pkg(dep, "/var/cache/pacman/pkg")
             if not dep_fullpath:
                 log_print(
@@ -2030,6 +2050,22 @@ def prepare_user_chroot(other_state: [str, Any]):
     return True
 
 
+def prefetch_dependency(pkg_name, other_state: [str, Any]):
+    """Returns "fetched" on success."""
+    log_print(
+        f'Prefetching package "{pkg_name}" with "pacman -Sw"...',
+        other_state=other_state,
+    )
+    try:
+        subprocess.run(
+            ("/usr/bin/env", "sudo", "pacman", "--noconfirm", "-Sw", pkg_name),
+            check=True,
+        )
+    except subprocess.CalledProcessError:
+        return "fail"
+    return "fetched"
+
+
 def main():
     """The main function."""
     signal.signal(signal.SIGINT, signal_handler)