Fix auto-update thrash: change root .gitattributes

.gitattributes

@@ -14,3 +14,28 @@
 # See golang.org/issue/9281.
 
 * -text
+
+# NO MICROSOFT_UPSTREAM:
+# The above lines in this file are the entire content of upstream's
+# ".gitattributes" file. It's important that the development environment in the
+# microsoft/go repo matches upstream's to avoid disrupting work on
+# contributions.
+#
+# The below lines apply autocrlf to a few specific files and folders to make
+# them work nicely with dotnet/arcade's auto-update infrastructure. These
+# files/folders aren't present in upstream.
+#
+# "darc update" is the auto-update command for dotnet/arcade, and it updates
+# file content in a way that ends up with CRLF on Windows and LF on Linux. This
+# can cause thrashing when checked in without autocrlf. We fix this by adding
+# normalization that matches the way Arcade expects it to work:
+# https://github.com/dotnet/arcade/blob/ec35b4879466a6071c16db8fa3ce5d2dca0dfbe8/.gitattributes
+
+/eng/common/** text=auto encoding=UTF-8
+/eng/Version.Details.xml text=auto encoding=UTF-8
+/eng/Versions.props text=auto encoding=UTF-8
+/global.json text=auto encoding=UTF-8
+/NuGet.config text=auto encoding=UTF-8
+
+/eng/common/**/*.sh text eol=lf
+# END NO MICROSOFT_UPSTREAM

eng/README.md

@@ -52,6 +52,21 @@ infrastructure requirements:
   SDK auto-update requires this file to be in this absolute location.
 * `/NuGet.config` - This is a .NET NuGet sources configuration file. This is
   also required at the root of the repo by the Arcade SDK.
+* `/.gitattributes` - When Arcade auto-update changes files, it uses CRLF or LF
+  depending on runtime/platform. The Go repo disables autocrlf, so this behavior
+  causes thrashing. To fix this, we modified the attributes file to turn
+  autocrlf back on for specific auto-updated files.
+
+To find TODO-style comments describing intentional changes to upstream files
+that seem suitable to contribute, search the repo for:
+
+```
+MICROSOFT_UPSTREAM
+```
+
+You will also find `NO MICROSOFT_UPSTREAM` marking changes that wouldn't be
+useful to contribute to upstream. Typically, changes marked this way have no
+effect whatsoever outside the context of the Microsoft-specific infrastructure.
 
 For a complete list of files that are modified vs. the upstream Git repository,
 first make sure you have the upstream Git refs locally. One way to do this is to

eng/_util/cmd/sync/sync.go

@@ -49,6 +49,7 @@ default. This avoids trampling changes in the user's clone.`
 //
 // This is in package scope just so it's easy to find at the top of the file for maintenance.
 var autoResolveOurPaths = []string{
+	".gitattributes",
 	".github",
 	"CODE_OF_CONDUCT.md",
 	"README.md",