From a12432dc94b54b6ff9e0b53607859ed6bd7141ca Mon Sep 17 00:00:00 2001
From: Guido van Rossum <guido@dropbox.com>
Date: Sat, 29 Apr 2017 21:13:19 -0700
Subject: [PATCH 1/3] Catch and ignore errors writing JSON files.

This is an alternative attempt at fixing issue #3215, given the
complexity of PR #3239.
---
 mypy/build.py | 32 +++++++++++++++++++++-----------
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/mypy/build.py b/mypy/build.py
index 5ab87ab323f0..6f3471581173 100644
--- a/mypy/build.py
+++ b/mypy/build.py
@@ -897,12 +897,17 @@ def write_cache(id: str, path: str, tree: MypyFile,
         data_mtime = os.path.getmtime(data_json)
         manager.trace("Interface for {} is unchanged".format(id))
     else:
-        with open(data_json_tmp, 'w') as f:
-            f.write(data_str)
-            f.write('\n')
-        data_mtime = os.path.getmtime(data_json_tmp)
-        os.replace(data_json_tmp, data_json)
         manager.trace("Interface for {} has changed".format(id))
+        try:
+            with open(data_json_tmp, 'w') as f:
+                f.write(data_str)
+                f.write('\n')
+            os.replace(data_json_tmp, data_json)
+            data_mtime = os.path.getmtime(data_json)
+        except os.error as err:
+            # Most likely the error is the replace() call
+            # (see https://github.com/python/mypy/issues/3215).
+            manager.log("Error writing data JSON file {}".format(data_json_tmp))
 
     mtime = st.st_mtime
     size = st.st_size
@@ -922,12 +927,17 @@ def write_cache(id: str, path: str, tree: MypyFile,
             }
 
     # Write meta cache file
-    with open(meta_json_tmp, 'w') as f:
-        if manager.options.debug_cache:
-            json.dump(meta, f, indent=2, sort_keys=True)
-        else:
-            json.dump(meta, f)
-    os.replace(meta_json_tmp, meta_json)
+    try:
+        with open(meta_json_tmp, 'w') as f:
+            if manager.options.debug_cache:
+                json.dump(meta, f, indent=2, sort_keys=True)
+            else:
+                json.dump(meta, f)
+        os.replace(meta_json_tmp, meta_json)
+    except os.error as err:
+        # Most likely the error is the replace() call
+        # (see https://github.com/python/mypy/issues/3215).
+        manager.log("Error writing meta JSON file {}".format(meta_json_tmp))
 
     return interface_hash
 

From be30f8b473721c4b0aae07c49be4e3b6c6382af2 Mon Sep 17 00:00:00 2001
From: Guido van Rossum <guido@dropbox.com>
Date: Sat, 29 Apr 2017 21:23:25 -0700
Subject: [PATCH 2/3] Bail early when writing the data file failes

---
 mypy/build.py | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mypy/build.py b/mypy/build.py
index 6f3471581173..5380409fa7a9 100644
--- a/mypy/build.py
+++ b/mypy/build.py
@@ -882,6 +882,7 @@ def write_cache(id: str, path: str, tree: MypyFile,
     except OSError as err:
         manager.log("Cannot get stat for {}: {}".format(path, err))
         # Remove apparently-invalid cache files.
+        # (This is purely an optimization.)
         for filename in [data_json, meta_json]:
             try:
                 os.remove(filename)
@@ -908,6 +909,15 @@ def write_cache(id: str, path: str, tree: MypyFile,
             # Most likely the error is the replace() call
             # (see https://github.com/python/mypy/issues/3215).
             manager.log("Error writing data JSON file {}".format(data_json_tmp))
+            # Let's continue without writing the meta file.  Analysis:
+            # If the replace failed, we've changed nothing except left
+            # behind an extraneous temporary file; if the replace
+            # worked but the getmtime() call failed, the meta file
+            # will be considered invalid on the next run because the
+            # data_mtime field won't match the data file's mtime.
+            # Both have the effect of slowing down the next run a
+            # little bit due to an out-of-date cache file.
+            return interface_hash
 
     mtime = st.st_mtime
     size = st.st_size
@@ -937,6 +947,7 @@ def write_cache(id: str, path: str, tree: MypyFile,
     except os.error as err:
         # Most likely the error is the replace() call
         # (see https://github.com/python/mypy/issues/3215).
+        # The next run will simply find the cache entry out of date.
         manager.log("Error writing meta JSON file {}".format(meta_json_tmp))
 
     return interface_hash

From 743e012d7111f529f39748baa332e8d7c8dde7a3 Mon Sep 17 00:00:00 2001
From: Guido van Rossum <guido@dropbox.com>
Date: Tue, 2 May 2017 09:56:13 -0700
Subject: [PATCH 3/3] Add a clarification to the docstring.

---
 mypy/build.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mypy/build.py b/mypy/build.py
index 5380409fa7a9..af1c6b474ca0 100644
--- a/mypy/build.py
+++ b/mypy/build.py
@@ -839,6 +839,10 @@ def write_cache(id: str, path: str, tree: MypyFile,
                 old_interface_hash: str, manager: BuildManager) -> str:
     """Write cache files for a module.
 
+    Note that this mypy's behavior is still correct when any given
+    write_cache() call is replaced with a no-op, so error handling
+    code that bails without writing anything is okay.
+
     Args:
       id: module ID
       path: module path