Add support for dotnet SDK fallback folder

The dotnet SDK ships with various .NET Core and ASP.NET Core nuget
packages in a NuGetFallbackFolder.

This changes to try to find packages in that folder before looking in
other caches or downloading from source. In the event the package is
found here, the contents are copied or linked directly from that folder
instead of the user's cache.

src/Paket.Core/Dependencies/NuGet.fs

@@ -760,12 +760,14 @@ let private downloadAndExtractPackage(alternativeProjectRoot, root, isLocalOverr
     let nupkgName = packageName.ToString() + "." + version.ToString() + ".nupkg"
     let normalizedNupkgName = NuGetCache.GetPackageFileName packageName version
     let configResolved = config.Resolve root groupName packageName version includeVersionInPath
-    let targetFileName =
+    let targetFileName, isTargetInFallbackFolder =
         if not isLocalOverride then
-            NuGetCache.GetTargetUserNupkg packageName version
+            match NuGetCache.TryGetFallbackNupkg None packageName version with
+            | Some fileName -> fileName, true
+            | None -> NuGetCache.GetTargetUserNupkg packageName version, false
         else
             match configResolved.Path with
-            | Some p -> Path.Combine(p, nupkgName)
+            | Some p -> Path.Combine(p, nupkgName), false
             | None -> failwithf "paket.local in combination with storage:none is not supported"
 
     if isLocalOverride && not force then
@@ -984,12 +986,17 @@ let private downloadAndExtractPackage(alternativeProjectRoot, root, isLocalOverr
         | false, otherConfig ->
             otherConfig.Path |> Option.iter SymlinkUtils.delete
 
-            let! extractedUserFolder = ExtractPackageToUserFolder(targetFile.FullName, packageName, version, kind)
+            let! extractedFolder = async {
+                if isTargetInFallbackFolder then return Path.GetDirectoryName targetFileName
+                else return! ExtractPackageToUserFolder(targetFile.FullName, packageName, version, kind) }
+
             let! files = NuGetCache.CopyFromCache(otherConfig, targetFile.FullName, licenseFileName, packageName, version, force, detailed)
+
             let finalFolder =
                 match files with
                 | Some f -> f
-                | None -> extractedUserFolder
+                | None -> extractedFolder
+
             return targetFileName,finalFolder
     }
 

src/Paket.Core/Dependencies/NuGetCache.fs

@@ -331,6 +331,21 @@ let GetTargetUserNupkg (packageName:PackageName) (version:SemVerInfo) =
 let GetTargetUserToolsFolder (packageName:PackageName) (version:SemVerInfo) =
     DirectoryInfo(Path.Combine(Constants.UserNuGetPackagesFolder,".tools",packageName.CompareString,version.Normalize())).FullName
 
+let TryGetFallbackFolder () =
+    let dotnet = if isUnix then "dotnet" else "dotnet.exe"
+    ProcessHelper.tryFindFileOnPath dotnet |> Option.bind (fun fileName ->
+        let dotnetDir = Path.GetDirectoryName fileName
+        let fallbackDir = Path.Combine (dotnetDir, "sdk", "NuGetFallbackFolder")
+        if Directory.Exists fallbackDir then Some fallbackDir else None)
+
+let TryGetFallbackNupkg (fallbackFolderOverride:string option) (packageName:PackageName) (version:SemVerInfo) =
+    match fallbackFolderOverride |> Option.orElseWith TryGetFallbackFolder with
+    | Some folder ->
+        let normalizedNupkgName = GetPackageFileName packageName version
+        let fallbackFile = Path.Combine(folder, packageName.CompareString, version.Normalize(), normalizedNupkgName) |> FileInfo
+        if fallbackFile.Exists && fallbackFile.Length > 0L then Some fallbackFile.FullName else None
+    | None -> None
+
 /// Extracts the given package to the user folder
 let rec ExtractPackageToUserFolder(fileName:string, packageName:PackageName, version:SemVerInfo, kind:PackageResolver.ResolvedPackageKind) =
     async {