Keep legacy version compliance checks in place for non-SemVer2 versions

src/NuGetGallery.Core/NuGetVersionExtensions.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Text.RegularExpressions;
 using NuGet.Versioning;
 
 namespace NuGetGallery
@@ -22,9 +23,19 @@ public static string Normalize(string version)
 
     public static class NuGetVersionExtensions
     {
+        private const RegexOptions SemanticVersionRegexFlags = RegexOptions.Compiled | RegexOptions.IgnoreCase | RegexOptions.ExplicitCapture;
+        private static readonly Regex SemanticVersionRegex = new Regex(@"^(?<Version>\d+(\s*\.\s*\d+){0,3})(?<Release>-[a-z][0-9a-z-]*)?$", SemanticVersionRegexFlags);
+
         public static string ToNormalizedStringSafe(this NuGetVersion self)
         {
             return self != null ? self.ToNormalizedString() : String.Empty;
         }
+
+        public static bool IsValidVersionForLegacyClients(this NuGetVersion self)
+        {
+            var match = SemanticVersionRegex.Match(self.ToString().Trim());
+
+            return match.Success;
+        }
     }
 }

src/NuGetGallery.Core/Packaging/ManifestValidator.cs

@@ -76,6 +76,12 @@ private static IEnumerable<ValidationResult> ValidateCore(PackageMetadata packag
                     version));
             }
 
+            var versionValidationResult = ValidateVersionForLegacyClients(packageMetadata.Version);
+            if (versionValidationResult != null)
+            {
+                yield return versionValidationResult;
+            }
+
             // Check framework reference groups
             var frameworkReferenceGroups = packageMetadata.GetFrameworkReferenceGroups();
             if (frameworkReferenceGroups != null)
@@ -160,6 +166,26 @@ private static IEnumerable<ValidationResult> ValidateCore(PackageMetadata packag
             }
         }
 
+        /// <summary>
+        /// Checks whether the provided version is consumable by legacy 2.x clients,
+        /// which do not support a `.` in release labels, or release labels starting with numeric characters.
+        /// See also https://github.com/NuGet/NuGetGallery/issues/3226.
+        /// </summary>
+        /// <param name="version">The <see cref="NuGetVersion"/> to check for 2.x client compatibility.</param>
+        /// <returns>Returns a <see cref="ValidationResult"/> when non-compliant; otherwise <c>null</c>.</returns>
+        private static ValidationResult ValidateVersionForLegacyClients(NuGetVersion version)
+        {
+            if (!version.IsSemVer2 && !version.IsValidVersionForLegacyClients())
+            {
+                return new ValidationResult(string.Format(
+                    CultureInfo.CurrentCulture,
+                    CoreStrings.Manifest_InvalidVersion,
+                    version));
+            }
+
+            return null;
+        }
+
         private static ValidationResult ValidateDependencyVersion(NuGetVersion version)
         {
             if (version.HasMetadata)
@@ -170,7 +196,7 @@ private static ValidationResult ValidateDependencyVersion(NuGetVersion version)
                     version.ToFullString()));
             }
 
-            return null;
+            return ValidateVersionForLegacyClients(version);
         }
 
         private static IEnumerable<ValidationResult> CheckUrls(params string[] urls)

tests/NuGetGallery.Core.Facts/Packaging/ManifestValidatorFacts.cs

@@ -440,6 +440,16 @@ public void ReturnsNoErrorIfDependencyVersionIsSemVer200WithoutMetadataPart()
             Assert.Empty(GetErrors(nuspecStream));
         }
 
+        [Theory]
+        [InlineData("1.0.0-10")]
+        [InlineData("1.0.0--")]
+        public void ReturnsErrorIfNonSemVer2VersionIsNotCompliantWith2XClients(string version)
+        {
+            var nuspecStream = CreateNuspecStream(string.Format(NuSpecPlaceholderVersion, version));
+
+            Assert.Equal(new[] {String.Format(CoreStrings.Manifest_InvalidVersion, version)}, GetErrors(nuspecStream));
+        }
+
         [Fact]
         public void ReturnsErrorIfDependencyVersionIsSemVer200WithMetadataPart()
         {