Fall back on SOURCE_DATE_EPOCH if it exists (3.x) (#281)

* Reproducible from environment

* Exclude the SOURCE_DATE_EPOCH environment variable when running tests

* Reduce a bit more the code duplication

Author: fridrich

pom.xml

@@ -115,4 +115,22 @@
       <scope>test</scope>
     </dependency>
   </dependencies>
+
+  <build>
+    <pluginManagement>
+      <plugins>
+        <plugin>
+          <groupId>org.apache.maven.plugins</groupId>
+          <artifactId>maven-surefire-plugin</artifactId>
+          <configuration>
+            <redirectTestOutputToFile>true</redirectTestOutputToFile>
+            <excludedEnvironmentVariables>
+              <excludedEnvironmentVariable>SOURCE_DATE_EPOCH</excludedEnvironmentVariable>
+            </excludedEnvironmentVariables>
+          </configuration>
+        </plugin>
+      </plugins>
+    </pluginManagement>
+  </build>
+
 </project>

src/main/java/org/apache/maven/archiver/MavenArchiver.java

@@ -755,9 +755,15 @@ public Date configureReproducible(String outputTimestamp) {
      *             section 4.4.6.
      */
     public static Optional<Instant> parseBuildOutputTimestamp(String outputTimestamp) {
-        // Fail-fast on nulls
-        if (outputTimestamp == null) {
-            return Optional.empty();
+        final String sourceDateEpoch = System.getenv("SOURCE_DATE_EPOCH");
+        // Fail fast on null and no timestamp configured (1 character configuration is useful to override
+        // a full value during pom inheritance)
+        if (outputTimestamp == null || (outputTimestamp.length() < 2 && !isNumeric(outputTimestamp))) {
+            if (sourceDateEpoch == null) {
+                return Optional.empty();
+            } else {
+                outputTimestamp = sourceDateEpoch;
+            }
         }
 
         // Number representing seconds since the epoch
@@ -771,12 +777,6 @@ public static Optional<Instant> parseBuildOutputTimestamp(String outputTimestamp
             return Optional.of(date);
         }
 
-        // no timestamp configured (1 character configuration is useful to override a full value during pom
-        // inheritance)
-        if (outputTimestamp.length() < 2) {
-            return Optional.empty();
-        }
-
         try {
             // Parse the date in UTC such as '2011-12-03T10:15:30Z' or with an offset '2019-10-05T20:37:42+06:00'.
             final Instant date = OffsetDateTime.parse(outputTimestamp)