develop: Allow shells to declare the minimal bash version they require

Allow derivations to export a `NIX_ENV_MIN_BASH_VERSION` that sets the
minimal (major) bash version that is required to parse their setup
script, and use that to gracefully fail if the current bash is too old.

Fix #10263 (from the Nix side at least, needs NixOS/nixpkgs#299490 from the Nixpkgs side to be useful in practice)

src/nix/develop.cc

@@ -54,6 +54,7 @@ struct BuildEnvironment
     std::map<std::string, Value> vars;
     std::map<std::string, std::string> bashFunctions;
     std::optional<std::pair<std::string, std::string>> structuredAttrs;
+    std::optional<int> minimalBashVersion;
 
     static BuildEnvironment fromJSON(std::string_view in)
     {
@@ -81,6 +82,10 @@ struct BuildEnvironment
             res.structuredAttrs = {json["structuredAttrs"][".attrs.json"], json["structuredAttrs"][".attrs.sh"]};
         }
 
+        if (json.contains("meta") && json["meta"].contains("min-bash-version")) {
+            res.minimalBashVersion = std::optional<int>{ json["meta"]["min-bash-version"] };
+        }
+
         return res;
     }
 
@@ -142,6 +147,18 @@ struct BuildEnvironment
 
     void toBash(std::ostream & out, const std::set<std::string> & ignoreVars) const
     {
+        if (minimalBashVersion.has_value()) {
+            out << stripIndentation(fmt(R"__NIX_STR(
+                            if [[ -n "${BASH_VERSINFO-}" && "${BASH_VERSINFO-}" -lt %d ]]; then
+                                echo "Detected Bash version that isn't supported by this derivation (${BASH_VERSION})"
+                                echo "Please install Bash %d or greater to continue."
+                                exit 1
+                            fi
+                        )__NIX_STR",
+                        minimalBashVersion.value(),
+                        minimalBashVersion.value()));
+        }
+
         for (auto & [name, value] : vars) {
             if (!ignoreVars.count(name)) {
                 if (auto str = std::get_if<String>(&value)) {

src/nix/get-env.sh

@@ -115,6 +115,16 @@ __dumpEnv() {
         printf '\n  }'
     fi
 
+    printf ',\n  "meta": {\n    '
+    # min-bash-version is supposed to be an integer.
+    # If not, just ignore it.
+    if [[ -n "${NIX_ENV_MIN_BASH_VERSION-}" && "$NIX_ENV_MIN_BASH_VERSION" -eq "$NIX_ENV_MIN_BASH_VERSION" ]]; then
+        __escapeString "min-bash-version"
+        printf ': '
+        printf '%d' "${NIX_ENV_MIN_BASH_VERSION}"
+    fi
+    printf '\n  }'
+
     printf '\n}'
 }
 

tests/functional/flakes/develop.sh

@@ -65,3 +65,20 @@ EOF
 )" -ef "$BASH_INTERACTIVE_EXECUTABLE" ]]
 
 clearStore
+
+# Use a newer bash version than the current one
+cat <<EOF >$TEST_HOME/flake.nix
+{
+    inputs.nixpkgs.url = "$TEST_HOME/nixpkgs";
+    outputs = {self, nixpkgs}: {
+      packages.$system.hello = (import ./config.nix).mkDerivation {
+        name = "hello";
+        outputs = [ "out" "dev" ];
+        meta.outputsToInstall = [ "out" ];
+        buildCommand = "";
+        NIX_ENV_MIN_BASH_VERSION=99;
+      };
+    };
+}
+EOF
+expect 1 nix develop .#hello --command true