Fix SLF4J conflict with Kotlin scripting, default log level (#203)

In #178, logging was changed to SLF4J and logback-classic. To keep it
possible to set the log level via environment variables
(`Config.logLevel` / `DEVELOCITY_API_LOG_LEVEL`), a small hack was in
place. Kotlin scripts, [which have `slf4j-simple` in the classpath by
default][1], crashed due to this hack.

```
java.lang.ClassCastException: class org.slf4j.impl.SimpleLogger cannot be cast to class ch.qos.logback.classic.Logger (org.slf4j.impl.SimpleLogger is in unnamed module of loader java.net.URLClassLoader @3a0f0552; ch.qos.logback.classic.Logger is in unnamed module of loader java.net.URLClassLoader @47b530e0)
        at com.gabrielfeo.develocity.api.internal.RealLoggerFactory.newLogger(LoggerFactory.kt:17)
        at com.gabrielfeo.develocity.api.internal.OkHttpClientKt.addNetworkInterceptors(OkHttpClient.kt:61)
        at com.gabrielfeo.develocity.api.internal.OkHttpClientKt.buildOkHttpClient(OkHttpClient.kt:35)
        at com.gabrielfeo.develocity.api.RealDevelocityApi$okHttpClient$2.invoke(DevelocityApi.kt:72)
        at com.gabrielfeo.develocity.api.RealDevelocityApi$okHttpClient$2.invoke(DevelocityApi.kt:71)
        at kotlin.SynchronizedLazyImpl.getValue(LazyJVM.kt:74)
        at com.gabrielfeo.develocity.api.RealDevelocityApi.getOkHttpClient(DevelocityApi.kt:71)
        at com.gabrielfeo.develocity.api.RealDevelocityApi.access$getOkHttpClient(DevelocityApi.kt:67)
        at com.gabrielfeo.develocity.api.RealDevelocityApi$retrofit$2.invoke(DevelocityApi.kt:78)
        at com.gabrielfeo.develocity.api.RealDevelocityApi$retrofit$2.invoke(DevelocityApi.kt:75)
        at kotlin.SynchronizedLazyImpl.getValue(LazyJVM.kt:74)
        at com.gabrielfeo.develocity.api.RealDevelocityApi.getRetrofit(DevelocityApi.kt:75)
        at com.gabrielfeo.develocity.api.RealDevelocityApi.access$getRetrofit(DevelocityApi.kt:67)
        at com.gabrielfeo.develocity.api.RealDevelocityApi$buildsApi$2.invoke(DevelocityApi.kt:84)
        at com.gabrielfeo.develocity.api.RealDevelocityApi$buildsApi$2.invoke(DevelocityApi.kt:84)
        at kotlin.SynchronizedLazyImpl.getValue(LazyJVM.kt:74)
        at com.gabrielfeo.develocity.api.RealDevelocityApi.getBuildsApi(DevelocityApi.kt:84)
        at Example_script_main$builds$1.invokeSuspend(example-script.main.kts:39)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:104)
        at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:277)
        at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:95)
        at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:69)
        at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
        at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:48)
        at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
        at Example_script_main.<init>(example-script.main.kts:38)
```

Also, the default log level was 'info', but it was supposed to be 'off'.

- Remove logback and any reference to SLF4J impls
- Add `slf4j-simple` instead with a simpler hack ✨ to set the log level
from environment variables
- Fix the default log level to be 'off', unless it'd conflict with the
user's own SimpleLogger default
- Document what the llbrary logs on each log level

[1]:
https://github.com/JetBrains/kotlin/blob/27c6d00cb2a5f9c0e9df62b021600e1263f5201a/libraries/tools/kotlin-main-kts/build.gradle.kts#L44

Author: gabrielfeo

library/build.gradle.kts

@@ -43,7 +43,7 @@ dependencies {
     implementation("com.squareup.retrofit2:converter-scalars:2.11.0")
     api("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.8.0")
     implementation("org.slf4j:slf4j-api:2.0.11")
-    implementation("ch.qos.logback:logback-classic:1.4.14")
+    runtimeOnly("org.slf4j:slf4j-simple:2.0.11")
     testImplementation("com.squareup.okhttp3:mockwebserver:4.12.0")
     testImplementation("com.squareup.okio:okio:3.9.0")
     testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.8.0")

library/src/main/kotlin/com/gabrielfeo/develocity/api/Config.kt

@@ -14,14 +14,23 @@ import kotlin.time.Duration.Companion.days
 data class Config(
 
     /**
-     * Forces a log level for internal library classes. By default, the library includes
-     * logback-classic with logging disabled, aimed at notebook and script usage. Logging may be
-     * enabled for troubleshooting by changing log level to "INFO", "DEBUG", etc.
+     * Changes the default log level for library classes, such as the HTTP client. By default,
+     * log level is the value of
+     * [org.slf4j.simpleLogger.defaultLogLevel](https://www.slf4j.org/api/org/slf4j/simple/SimpleLogger.html)
+     * system property or `"off"`.
      *
-     * To use different SLF4J bindings, simply exclude the logback dependency.
+     * Possible values:
+     * - "off" (default)
+     * - "error"
+     * - "warn"
+     * - "info"
+     * - "debug" (logs HTTP traffic: URLs and status codes only)
+     * - "trace" (logs HTTP traffic: full request and response including body, excluding
+     *   authorization header)
      */
-    val logLevel: String? =
-        env["DEVELOCITY_API_LOG_LEVEL"],
+    val logLevel: String =
+        env["DEVELOCITY_API_LOG_LEVEL"]
+            ?: "off",
 
     /**
      * Provides the URL of a Develocity API instance REST API. By default, uses

library/src/main/kotlin/com/gabrielfeo/develocity/api/internal/LoggerFactory.kt

@@ -1,6 +1,5 @@
 package com.gabrielfeo.develocity.api.internal
 
-import ch.qos.logback.classic.Level
 import com.gabrielfeo.develocity.api.Config
 import org.slf4j.Logger
 import kotlin.reflect.KClass
@@ -14,9 +13,16 @@ class RealLoggerFactory(
 ) : LoggerFactory {
 
     override fun newLogger(cls: KClass<*>): Logger {
-        val impl = org.slf4j.LoggerFactory.getLogger(cls.java) as ch.qos.logback.classic.Logger
-        return impl.apply {
-            level = Level.valueOf(config.logLevel)
+        setLogLevel()
+        return org.slf4j.LoggerFactory.getLogger(cls.java)
+    }
+
+    private fun setLogLevel() {
+        if (System.getProperty(SIMPLE_LOGGER_LOG_LEVEL) != null) {
+            return
         }
+        System.setProperty(SIMPLE_LOGGER_LOG_LEVEL, config.logLevel)
     }
 }
+
+internal const val SIMPLE_LOGGER_LOG_LEVEL = "org.slf4j.simpleLogger.defaultLogLevel"

library/src/main/kotlin/com/gabrielfeo/develocity/api/internal/OkHttpClient.kt

@@ -58,19 +58,12 @@ private fun OkHttpClient.Builder.addNetworkInterceptors(
     if (config.cacheConfig.cacheEnabled) {
         addNetworkInterceptor(buildCacheEnforcingInterceptor(config))
     }
-    val httpLogger = loggerFactory.newLogger(HttpLoggingInterceptor::class)
-    getHttpLoggingInterceptorForLogger(httpLogger)?.let {
-        addNetworkInterceptor(it)
-    }
+    val logger = loggerFactory.newLogger(HttpLoggingInterceptor::class)
+    addNetworkInterceptor(HttpLoggingInterceptor(logger = logger::debug).apply { level = BASIC })
+    addNetworkInterceptor(HttpLoggingInterceptor(logger = logger::trace).apply { level = BODY })
     addNetworkInterceptor(HttpBearerAuth("bearer", config.apiToken()))
 }
 
-private fun getHttpLoggingInterceptorForLogger(logger: Logger): Interceptor? = when {
-    logger.isDebugEnabled -> HttpLoggingInterceptor(logger = logger::debug).apply { level = BASIC }
-    logger.isTraceEnabled -> HttpLoggingInterceptor(logger = logger::debug).apply { level = BODY }
-    else -> null
-}
-
 internal fun buildCache(
     config: Config,
     loggerFactory: LoggerFactory,

library/src/test/kotlin/com/gabrielfeo/develocity/api/internal/LoggerFactoryTest.kt

@@ -0,0 +1,39 @@
+package com.gabrielfeo.develocity.api.internal
+
+import kotlin.test.Test
+import com.gabrielfeo.develocity.api.Config
+import kotlin.test.AfterTest
+import kotlin.test.BeforeTest
+import kotlin.test.assertEquals
+
+class LoggerFactoryTest {
+
+    @BeforeTest
+    @AfterTest
+    fun cleanup() {
+        System.clearProperty(SIMPLE_LOGGER_LOG_LEVEL)
+        env = FakeEnv("DEVELOCITY_API_URL" to "https://example.com/")
+    }
+
+    @Test
+    fun `Logging off by default`() {
+        val loggerFactory = RealLoggerFactory(Config())
+        loggerFactory.newLogger(LoggerFactoryTest::class)
+        assertEquals("off", System.getProperty(SIMPLE_LOGGER_LOG_LEVEL))
+    }
+
+    @Test
+    fun `Pre-existing defaultLogLevel is honored`() {
+        val loggerFactory = RealLoggerFactory(Config(logLevel = "bar"))
+        System.setProperty(SIMPLE_LOGGER_LOG_LEVEL, "foo")
+        loggerFactory.newLogger(LoggerFactoryTest::class)
+        assertEquals("foo", System.getProperty(SIMPLE_LOGGER_LOG_LEVEL))
+    }
+
+    @Test
+    fun `Logging can be set from config`() {
+        val loggerFactory = RealLoggerFactory(Config(logLevel = "foo"))
+        loggerFactory.newLogger(LoggerFactoryTest::class)
+        assertEquals("foo", System.getProperty(SIMPLE_LOGGER_LOG_LEVEL))
+    }
+}