This is because DigestInputStream implements the method in the following way.

    public int read(byte[] b, int off, int len) throws IOException {
        int result = in.read(b, off, len);
        if (on && result != -1) {
            digest.update(b, off, result);
        }
        return result;
    }

Code changed in jenkins
User: Christoph Kutzinski
Path:
changelog.html
core/src/main/java/hudson/Util.java
core/src/test/java/hudson/UtilTest.java
http://jenkins-ci.org/commit/jenkins/3c09a0c43ac85c2f85a6fd25fc12c2ab357f1461
Log:
[FIXED JENKINS-10346] thread-safety problem when computing fingerprints

Integrated in jenkins_main_trunk #956
[FIXED JENKINS-10346] thread-safety problem when computing fingerprints

Kohsuke Kawaguchi : 3c09a0c43ac85c2f85a6fd25fc12c2ab357f1461
Files :

• core/src/test/java/hudson/UtilTest.java
• core/src/main/java/hudson/Util.java
• changelog.html

Code changed in jenkins
User: Christoph Kutzinski
Path:
changelog.html
core/src/main/java/hudson/Util.java
core/src/test/java/hudson/UtilTest.java
http://jenkins-ci.org/commit/jenkins/e436a288cb37c7ce0a635026598334f87e6e63e6
Log:
[FIXED JENKINS-10346] thread-safety problem when computing fingerprints
(cherry picked from commit 3c09a0c43ac85c2f85a6fd25fc12c2ab357f1461)

Conflicts:

changelog.html

Code changed in jenkins
User: Christoph Kutzinski
Path:
changelog.html
core/src/main/java/hudson/Util.java
core/src/test/java/hudson/UtilTest.java
http://jenkins-ci.org/commit/jenkins/3c09a0c43ac85c2f85a6fd25fc12c2ab357f1461
Log:
[FIXED JENKINS-10346] thread-safety problem when computing fingerprints

Code changed in jenkins
User: Christoph Kutzinski
Path:
changelog.html
core/src/main/java/hudson/Util.java
core/src/test/java/hudson/UtilTest.java
http://jenkins-ci.org/commit/jenkins/3c09a0c43ac85c2f85a6fd25fc12c2ab357f1461
Log:
[FIXED JENKINS-10346] thread-safety problem when computing fingerprints