fix: silence SLF4J NOP-logger warning from JGit in serve command

MODULE.bazel

@@ -61,6 +61,12 @@ maven.install(
         "org.apache.commons:commons-pool2:2.11.1",
         "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.5.2",
         "org.mockito.kotlin:mockito-kotlin:5.4.0",
+        # SLF4J binding for JGit (serve command). JGit depends on slf4j-api but ships no binding, so
+        # without one SLF4J prints a "Failed to load class StaticLoggerBinder / Defaulting to NOP"
+        # warning to stderr on first use. slf4j-nop supplies the binding (silencing the warning) and
+        # discards JGit's internal logs, keeping the CLI's stderr clean. Version tracks the slf4j-api
+        # that JGit 5.13.x resolves (1.7.30).
+        "org.slf4j:slf4j-nop:1.7.30",
     ],
     fail_if_repin_required = True,
     lock_file = "//:maven_install.json",

cli/BUILD

@@ -41,6 +41,12 @@ kt_jvm_library(
         "@bazel_diff_maven//:org_eclipse_jgit_org_eclipse_jgit",
         "@bazel_diff_maven//:org_jetbrains_kotlinx_kotlinx_coroutines_core_jvm",
     ],
+    # SLF4J binding for JGit. Not referenced by bazel-diff code (JGit uses it internally), so it is a
+    # pure runtime dep: it supplies org.slf4j.impl.StaticLoggerBinder, silencing the "Defaulting to
+    # NOP logger" warning SLF4J prints to stderr when no binding is on the classpath.
+    runtime_deps = [
+        "@bazel_diff_maven//:org_slf4j_slf4j_nop",
+    ],
 )
 
 kt_jvm_test(
@@ -169,6 +175,12 @@ kt_jvm_test(
     runtime_deps = [":cli-test-lib"],
 )
 
+kt_jvm_test(
+    name = "Slf4jBindingTest",
+    test_class = "com.bazel_diff.log.Slf4jBindingTest",
+    runtime_deps = [":cli-test-lib"],
+)
+
 kt_jvm_test(
     name = "BazelModServiceTest",
     test_class = "com.bazel_diff.bazel.BazelModServiceTest",
@@ -273,6 +285,8 @@ kt_jvm_library(
         "@bazel_diff_maven//:io_insert_koin_koin_test_jvm",
         "@bazel_diff_maven//:junit_junit",
         "@bazel_diff_maven//:org_mockito_kotlin_mockito_kotlin",
+        # Compile-time API for Slf4jBindingTest; the runtime binding (slf4j-nop) comes via cli-lib.
+        "@bazel_diff_maven//:org_slf4j_slf4j_api",
     ],
 )
 

cli/src/test/kotlin/com/bazel_diff/log/Slf4jBindingTest.kt

@@ -0,0 +1,61 @@
+package com.bazel_diff.log
+
+import assertk.assertThat
+import assertk.assertions.doesNotContain
+import assertk.assertions.isNotNull
+import java.io.ByteArrayOutputStream
+import java.io.PrintStream
+import org.junit.After
+import org.junit.Before
+import org.junit.Test
+import org.slf4j.LoggerFactory
+
+/**
+ * Regression test for the SLF4J warning printed by `bazel-diff serve`:
+ * ```
+ * SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
+ * SLF4J: Defaulting to no-operation (NOP) logger implementation
+ * ```
+ *
+ * JGit (the serve command's in-process git engine) depends on `slf4j-api` but ships no binding. With
+ * no `org.slf4j.impl.StaticLoggerBinder` on the classpath, SLF4J falls back to a NOP logger and
+ * prints the warning above to stderr on first use. Shipping `slf4j-nop` as a runtime dep supplies the
+ * binding, silencing the warning while discarding JGit's internal logs (keeping the CLI stderr clean).
+ */
+class Slf4jBindingTest {
+  private val originalErr = System.err
+  private lateinit var captured: ByteArrayOutputStream
+
+  @Before
+  fun setUp() {
+    captured = ByteArrayOutputStream()
+    System.setErr(PrintStream(captured))
+  }
+
+  @After
+  fun tearDown() {
+    System.setErr(originalErr)
+  }
+
+  @Test
+  fun staticLoggerBinderIsOnTheClasspath() {
+    // This is the exact class SLF4J fails to load when no binding is present ("Failed to load class
+    // org.slf4j.impl.StaticLoggerBinder"). If it resolves, the NOP-fallback warning cannot be
+    // printed — a deterministic guard that survives SLF4J's once-per-JVM static init ordering.
+    val binder = Class.forName("org.slf4j.impl.StaticLoggerBinder")
+    assertThat(binder).isNotNull()
+  }
+
+  @Test
+  fun obtainingAndUsingALoggerDoesNotPrintNopFallbackWarning() {
+    // Exercises the same path JGit takes: resolve a logger and emit a record. With a binding present
+    // this is silent; without one, SLF4J's static init prints the warning to stderr.
+    val logger = LoggerFactory.getLogger(Slf4jBindingTest::class.java)
+    assertThat(logger).isNotNull()
+    logger.info("bazel-diff slf4j binding smoke test")
+
+    val err = captured.toString()
+    assertThat(err).doesNotContain("StaticLoggerBinder")
+    assertThat(err).doesNotContain("NOP logger")
+  }
+}

maven_install.json

@@ -12,6 +12,7 @@
     "org.eclipse.jgit:org.eclipse.jgit": -1935467463,
     "org.jetbrains.kotlinx:kotlinx-coroutines-core": -542524036,
     "org.mockito.kotlin:mockito-kotlin": 1836434344,
+    "org.slf4j:slf4j-nop": 1480452039,
     "repositories": -1949687017
   },
   "__RESOLVED_ARTIFACTS_HASH": {
@@ -52,7 +53,8 @@
     "org.mockito:mockito-core": -960687581,
     "org.objenesis:objenesis": 1798216877,
     "org.opentest4j:opentest4j": -1584531193,
-    "org.slf4j:slf4j-api": -801231047
+    "org.slf4j:slf4j-api": -801231047,
+    "org.slf4j:slf4j-nop": -2006403554
   },
   "conflict_resolution": {
     "io.insert-koin:koin-core-jvm:3.1.6": "io.insert-koin:koin-core-jvm:4.0.0"
@@ -285,6 +287,12 @@
         "jar": "cdba07964d1bb40a0761485c6b1e8c2f8fd9eb1d19c53928ac0d7f9510105c57"
       },
       "version": "1.7.30"
+    },
+    "org.slf4j:slf4j-nop": {
+      "shasums": {
+        "jar": "2d550dcefaea23d223b72027dbc7cbdb7327676ccefdd9cfe49cf9ea8e9ac8e0"
+      },
+      "version": "1.7.30"
     }
   },
   "dependencies": {
@@ -371,6 +379,9 @@
       "net.bytebuddy:byte-buddy",
       "net.bytebuddy:byte-buddy-agent",
       "org.objenesis:objenesis"
+    ],
+    "org.slf4j:slf4j-nop": [
+      "org.slf4j:slf4j-api"
     ]
   },
   "packages": {
@@ -887,6 +898,9 @@
       "org.slf4j.event",
       "org.slf4j.helpers",
       "org.slf4j.spi"
+    ],
+    "org.slf4j:slf4j-nop": [
+      "org.slf4j.impl"
     ]
   },
   "repositories": {
@@ -928,7 +942,8 @@
       "org.mockito:mockito-core",
       "org.objenesis:objenesis",
       "org.opentest4j:opentest4j",
-      "org.slf4j:slf4j-api"
+      "org.slf4j:slf4j-api",
+      "org.slf4j:slf4j-nop"
     ]
   },
   "services": {