Make sure that GetChildrenFromS3 returns correct results

Previously GetChildrenFromS3() returned children of the path and all
paths for which the given path was a prefix. This was because we didn't
append '/' to the end of the path, as S3 expects.

cloud/aws/aws_env.cc

@@ -844,6 +844,9 @@ Status AwsEnv::GetChildrenFromS3(const std::string& path,
 
   // S3 paths don't start with '/'
   auto prefix = ltrim_if(path, '/');
+  // S3 paths better end with '/', otherwise we might also get a list of files
+  // in a directory for which our path is a prefix
+  prefix = ensure_ends_with_pathsep(std::move(prefix));
   // the starting object marker
   Aws::String marker;
   bool loop = true;
@@ -884,7 +887,7 @@ Status AwsEnv::GetChildrenFromS3(const std::string& path,
       if (keystr.find(prefix) != 0) {
         return Status::IOError("Unexpected result from AWS S3: " + keystr);
       }
-      auto fname = ltrim_if(keystr.substr(prefix.size()), '/');
+      auto fname = keystr.substr(prefix.size());
       result->push_back(fname);
     }
 

cloud/filename.h

@@ -45,6 +45,15 @@ inline std::string dirname(std::string const& pathname) {
   return pathname.substr(0, pos);
 }
 
+// If s doesn't end with '/', it appends it.
+// Special case: if s is empty, we don't append '/'
+inline std::string ensure_ends_with_pathsep(std::string s) {
+    if (!s.empty() && s.back() != '/') {
+        s += '/';
+    }
+    return s;
+}
+
 // If the last char of the string is the specified character, then return a
 // string that has the last character removed.
 inline std::string rtrim_if(std::string s, char c) {