feat: add population of the SourceLocation from context (#721)

* feat: add population of the SourceLocation from context

* 🦉 Updates from OwlBot

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Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: minherz

google-cloud-logging/src/main/java/com/google/cloud/logging/SourceLocation.java

@@ -16,6 +16,8 @@
 
 package com.google.cloud.logging;
 
+import static com.google.common.base.Preconditions.checkElementIndex;
+
 import com.google.common.base.MoreObjects;
 import com.google.logging.v2.LogEntrySourceLocation;
 import java.io.Serializable;
@@ -154,4 +156,30 @@ static SourceLocation fromPb(LogEntrySourceLocation sourceLocationPb) {
         .setFunction(sourceLocationPb.getFunction())
         .build();
   }
+
+  /**
+   * Creates instance of {@link SourceLocation} based on stack trace information. Caller should
+   * provide the level in the stack where the information can be located. The stack trace level
+   * should be {@code 0} to display information for the caller of the method.
+   *
+   * @param level Zero-based non-negative integer defining the level in the stack trace where {@code
+   *     0} is topmost element.
+   * @return a new instance of {@link SourceLocation} populated with file name, method and line
+   *     number information.
+   * @throws IndexOutOfBoundsException if the provided {@link level} is negative or greater than the
+   *     current call stack.
+   */
+  static SourceLocation fromCurrentContext(int level) {
+    StackTraceElement[] stackTrace = (new Exception()).getStackTrace();
+    Builder builder = newBuilder();
+    // need to take info from 1 level down the stack to compensate the call to this
+    // method
+    int indexPlus = checkElementIndex(level, stackTrace.length - 1) + 1;
+    StackTraceElement ste = stackTrace[indexPlus];
+    return builder
+        .setFile(ste.getFileName())
+        .setLine(Long.valueOf(ste.getLineNumber()))
+        .setFunction(ste.getMethodName())
+        .build();
+  }
 }

google-cloud-logging/src/test/java/com/google/cloud/logging/SourceLocationTest.java

@@ -58,6 +58,27 @@ public void testToAndFromPb() {
     compareSourceLocation(SOURCE_LOCATION, SourceLocation.fromPb(SOURCE_LOCATION.toPb()));
   }
 
+  @Test
+  public void testFromCurrentContext() {
+    StackTraceElement expectedData = (new Exception()).getStackTrace()[0];
+    SourceLocation data = SourceLocation.fromCurrentContext(0);
+    assertEquals(expectedData.getFileName(), data.getFile());
+    assertEquals(expectedData.getMethodName(), data.getFunction());
+    // mind the assertion is vs (expectedData.lineNumber + 1). it is because the source location
+    // info of the expectedData is one line above the source location of the tested data.
+    assertEquals(Long.valueOf(expectedData.getLineNumber() + 1), data.getLine());
+  }
+
+  @Test(expected = IndexOutOfBoundsException.class)
+  public void testFromCurrentContextWithNegativeLevel() {
+    SourceLocation.fromCurrentContext(-1);
+  }
+
+  @Test(expected = IndexOutOfBoundsException.class)
+  public void testFromCurrentContextWithVeryLargeLevel() {
+    SourceLocation.fromCurrentContext(10000);
+  }
+
   private void compareSourceLocation(SourceLocation expected, SourceLocation value) {
     assertEquals(expected, value);
     assertEquals(expected.getFile(), value.getFile());