From ef4ca6a66f40b8807b1b48fc6a7292696cd7e1af Mon Sep 17 00:00:00 2001
From: Oscar Boykin <oscar@stripe.com>
Date: Wed, 27 Jul 2016 15:38:00 -1000
Subject: [PATCH 1/2] add some debug

---
 thrift/thrift.bzl | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/thrift/thrift.bzl b/thrift/thrift.bzl
index ae15e3c79..8518b1d30 100644
--- a/thrift/thrift.bzl
+++ b/thrift/thrift.bzl
@@ -23,6 +23,9 @@ rm -rf {out}_tmp"""
 
   cmd = cmd.format(out=ctx.outputs.libarchive.path,
                    jar=ctx.file._jar.path)
+  print(cmd)
+  print([f.path for f in ctx.files.srcs])
+
   ctx.action(
     inputs = ctx.files.srcs +
       ctx.files._jar +

From 3f786044d31dc701fd4f0fbf67ee278e9fda4ded Mon Sep 17 00:00:00 2001
From: Oscar Boykin <oscar@stripe.com>
Date: Thu, 28 Jul 2016 11:13:37 -1000
Subject: [PATCH 2/2] improve prefix operation

---
 thrift/thrift.bzl | 35 +++++++++++++++++++++++++++++++----
 1 file changed, 31 insertions(+), 4 deletions(-)

diff --git a/thrift/thrift.bzl b/thrift/thrift.bzl
index 8518b1d30..15ae8494f 100644
--- a/thrift/thrift.bzl
+++ b/thrift/thrift.bzl
@@ -2,11 +2,40 @@
 
 _thrift_filetype = FileType([".thrift"])
 
+def _common_prefix(strings):
+  pref = None
+  for s in strings:
+    if pref == None:
+      pref = s
+    elif s.startswith(pref):
+      pass
+    else:
+      tmp_pref = pref
+      for end in range(0, len(pref) + 1):
+        test = pref[0:end]
+        if s.startswith(test):
+          tmp_pref = test
+      pref = tmp_pref
+  return pref
+
 def _thrift_library_impl(ctx):
   prefix = ctx.attr.absolute_prefix
+  src_paths = [f.path for f in ctx.files.srcs]
+  if len(src_paths) <= 0:
+    fail("we require at least one thrift file in a target")
+
   jarcmd = "{jar} cMf {out} -C {out}_tmp ."
   if prefix != '':
-    jarcmd = "{{jar}} cMf {{out}} -C {{out}}_tmp/{prefix} .".format(prefix=prefix)
+    common_prefix = _common_prefix(src_paths)
+    pos = common_prefix.find(prefix)
+    if pos < 0:
+      fail("could not find prefix: {prefix} in the common prefix: {common_prefix}".format(
+          prefix = prefix,
+          common_prefix = common_prefix))
+    else:
+      endpos = pos + len(prefix)
+      actual_prefix = common_prefix[0:endpos]
+      jarcmd = "{{jar}} cMf {{out}} -C {{out}}_tmp/{pf} .".format(pf=actual_prefix)
 
   _valid_thrift_deps(ctx.attr.deps)
   # We move the files and touch them so that the output file is a purely deterministic
@@ -23,8 +52,6 @@ rm -rf {out}_tmp"""
 
   cmd = cmd.format(out=ctx.outputs.libarchive.path,
                    jar=ctx.file._jar.path)
-  print(cmd)
-  print([f.path for f in ctx.files.srcs])
 
   ctx.action(
     inputs = ctx.files.srcs +
@@ -33,7 +60,7 @@ rm -rf {out}_tmp"""
     outputs = [ctx.outputs.libarchive],
     command = cmd,
     progress_message = "making thrift archive %s" % ctx.label,
-    arguments = [f.path for f in ctx.files.srcs],
+    arguments = src_paths,
   )
 
   transitive_srcs = _collect_thrift_srcs(ctx.attr.deps)