Fix how the timestamps of add-source packages are updated.

This is to address #1375.

Previously they were updated in the post install phase, which
would mean that the sources did not have their timestamps
updated unless the package being installed completed successfully.

This change moves the updating of the timestamps to the end of
the install phase, so that the add-source packages that are
successfully installed get their timestamps updated regardless of
the overall outcome of the installation.

cabal-install/Distribution/Client/Install.hs

@@ -337,7 +337,7 @@ processInstallPlan verbosity
 
     unless (dryRun || nothingToInstall) $ do
       buildOutcomes <- performInstallations verbosity
-                         args installedPkgIndex installPlan
+                       args installedPkgIndex installPlan
       postInstallActions verbosity args userTargets installPlan buildOutcomes
   where
     installPlan = InstallPlan.configureInstallPlan configFlags installPlan0
@@ -821,6 +821,9 @@ postInstallActions verbosity
   ,globalFlags, configFlags, _, installFlags, _)
   targets installPlan buildOutcomes = do
 
+  updateSandboxTimestampsFile verbosity useSandbox mSandboxPkgInfo
+                              comp platform installPlan buildOutcomes
+
   unless oneShot $
     World.insert verbosity worldFile
       --FIXME: does not handle flags
@@ -846,9 +849,6 @@ postInstallActions verbosity
 
   printBuildFailures verbosity buildOutcomes
 
-  updateSandboxTimestampsFile verbosity useSandbox mSandboxPkgInfo
-                              comp platform installPlan buildOutcomes
-
   where
     reportingLevel = fromFlag (installBuildReports installFlags)
     logsDir        = fromFlag (globalLogsDir globalFlags)

cabal-install/changelog

@@ -38,8 +38,11 @@
 	* 'new-build' now checks for the existence of executables for
 	build-tools and build-tool-depends dependencies in the solver
 	(#4884).
-        * Fixed a spurious warning telling the user to run 'cabal update'
+	* Fixed a spurious warning telling the user to run 'cabal update'
 	when it wasn't necessary (#4444).
+	* Packages installed in sandboxes via 'add-source' now have
+	their timestamps updated correctly and so will not be reinstalled
+	unncecessarily if the main install command fails (#1375).
 
 2.0.0.1 Mikhail Glushenkov <mikhail.glushenkov@gmail.com> December 2017
 	* Support for GHC's numeric -g debug levels (#4673).