nix-shell: fix shebang whitespace parsing

Leading whitespace after `nix-shell` used to produce an empty argument,
while an empty argument at the end of the line was ignored.

Fix the first issue by consuming the initial whitespace before calling
shellwords; fix the second issue by returning immediately if whitespace
is found at the end of the string instead of checking for an empty
string.

Also throw if quotes aren't terminated.

src/nix-build/nix-build.cc

@@ -34,7 +34,7 @@ extern char * * environ __attribute__((weak));
  */
 static std::vector<std::string> shellwords(const std::string & s)
 {
-    std::regex whitespace("^(\\s+).*");
+    std::regex whitespace("^\\s+");
     auto begin = s.cbegin();
     std::vector<std::string> res;
     std::string cur;
@@ -51,9 +51,10 @@ static std::vector<std::string> shellwords(const std::string & s)
             if (regex_search(it, s.cend(), match, whitespace)) {
                 cur.append(begin, it);
                 res.push_back(cur);
-                cur.clear();
-                it = match[1].second;
+                it = match[0].second;
+                if (it == s.cend()) return res;
                 begin = it;
+                cur.clear();
             }
         }
         switch (*it) {
@@ -80,8 +81,9 @@ static std::vector<std::string> shellwords(const std::string & s)
                 break;
         }
     }
+    if (st != sBegin) throw Error("unterminated quote in shebang line");
     cur.append(begin, it);
-    if (!cur.empty()) res.push_back(cur);
+    res.push_back(cur);
     return res;
 }
 
@@ -140,7 +142,7 @@ static void main_nix_build(int argc, char * * argv)
                 for (auto line : lines) {
                     line = chomp(line);
                     std::smatch match;
-                    if (std::regex_match(line, match, std::regex("^#!\\s*nix-shell (.*)$")))
+                    if (std::regex_match(line, match, std::regex("^#!\\s*nix-shell\\s+(.*)$")))
                         for (const auto & word : shellwords(match[1].str()))
                             args.push_back(word);
                 }