libutil: add which()

Change-Id: I2bf80d09196b5c84891043e09b23e012fe6cfc4c

src/libutil/util.cc

@@ -1299,6 +1299,32 @@ void runProgram2(const RunOptions & options)
         throw ExecError(status, "program '%1%' %2%", options.program, statusToString(status));
 }
 
+std::optional<Path> which(std::string_view cmdName)
+{
+    for (auto const & dir : tokenizeString<StringSet>(getEnv("PATH").value_or(""), ":")) {
+        auto supposedExe = dir + "/" + cmdName;
+        if (pathExists(supposedExe)) {
+            return std::move(supposedExe);
+        }
+    }
+
+    return std::nullopt;
+}
+
+std::vector<Path> whichAll(std::string_view cmdName)
+{
+    std::vector<Path> results;
+
+    for (auto const & dir : tokenizeString<StringSet>(getEnv("PATH").value_or(""), ":")) {
+        auto supposedExe = dir + "/" + cmdName;
+        if (pathExists(supposedExe)) {
+            results.push_back(std::move(supposedExe));
+        }
+    }
+
+    return results;
+}
+
 
 void closeMostFDs(const std::set<int> & exceptions)
 {

src/libutil/util.hh

@@ -521,6 +521,31 @@ public:
  */
 std::vector<char *> stringsToCharPtrs(const Strings & ss);
 
+/** Gets the first file called `cmdName` in colon-separated PATH.
+ *
+ * @param cmdName The name of the file to look for in PATH.
+ *
+ * @return std::nullopt if the specified filename was not found.
+ *
+ * @note This does *not* check if the found file is executable or not.
+ *
+ * @see whichAll()
+ */
+std::optional<Path> which(std::string_view cmdName);
+
+/** Gets all files called `cmdName` in colon-separated PATH.
+ *
+ * @param cmdName The name of the file to look for in PATH.
+ *
+ * @return an std::vector of @ref Path "Paths", which will be empty if
+ * none were found.
+ *
+ * @note This does *not* check if the found files are executable or not.
+ *
+ * @see which()
+ */
+std::vector<Path> whichAll(std::string_view cmdName);
+
 /**
  * Close all file descriptors except those listed in the given set.
  * Good practice in child processes.

src/nix/doctor.cc

@@ -77,9 +77,9 @@ struct CmdDoctor : StoreCommand
     {
         PathSet dirs;
 
-        for (auto & dir : tokenizeString<Strings>(getEnv("PATH").value_or(""), ":"))
-            if (pathExists(dir + "/nix-env"))
-                dirs.insert(dirOf(canonPath(dir + "/nix-env", true)));
+        for (auto const & nixExeInPath : whichAll("nix-env")) {
+            dirs.insert(canonPath(dirOf(nixExeInPath), true));
+        }
 
         if (dirs.size() != 1) {
             std::stringstream ss;

src/nix/upgrade-nix.cc

@@ -104,10 +104,8 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
     {
         Path where;
 
-        for (auto & dir : tokenizeString<Strings>(getEnv("PATH").value_or(""), ":"))
-            if (pathExists(dir + "/nix-env")) {
-                where = dir;
-                break;
+        if (auto nixEnvPath = which("nix-env")) {
+            where = dirOf(*nixEnvPath);
         }
 
         if (where == "")