Adjust token expiry window to 40 seconds because of Azure (#170)

## Changes
<!-- Summary of your changes that are easy to understand -->

- Align with databricks/databricks-sdk-py#392
- Add ClockSupplier so that we don't depend on system clock while
testing. Modify Token class to be more test friendly.

## Tests
<!-- How is this tested? -->
Unit Tests
All nightly tests passed except two tests (which failed due to Max limit
of 100 scopes has been reached!). It's a known current test infra issue
and unrelated to these changes.

Author: tanmay-db

databricks-sdk-java/src/main/java/com/databricks/sdk/core/ApiClient.java

@@ -4,7 +4,7 @@
 import com.databricks.sdk.core.http.HttpClient;
 import com.databricks.sdk.core.http.Request;
 import com.databricks.sdk.core.http.Response;
-import com.databricks.sdk.core.utils.RealTimer;
+import com.databricks.sdk.core.utils.SystemTimer;
 import com.databricks.sdk.core.utils.Timer;
 import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.core.JsonProcessingException;
@@ -46,7 +46,7 @@ public String configuredAccountID() {
   }
 
   public ApiClient(DatabricksConfig config) {
-    this(config, new RealTimer());
+    this(config, new SystemTimer());
   }
 
   public ApiClient(DatabricksConfig config, Timer timer) {

databricks-sdk-java/src/main/java/com/databricks/sdk/core/oauth/Token.java

@@ -1,5 +1,7 @@
 package com.databricks.sdk.core.oauth;
 
+import com.databricks.sdk.core.utils.ClockSupplier;
+import com.databricks.sdk.core.utils.SystemClockSupplier;
 import com.fasterxml.jackson.annotation.JsonProperty;
 import java.time.LocalDateTime;
 import java.time.temporal.ChronoUnit;
@@ -21,28 +23,50 @@ public class Token {
    */
   @JsonProperty private LocalDateTime expiry;
 
+  private final ClockSupplier clockSupplier;
+
   /** Constructor for non-refreshable tokens (e.g. M2M). */
   public Token(String accessToken, String tokenType, LocalDateTime expiry) {
-    this(accessToken, tokenType, null, expiry);
+    this(accessToken, tokenType, null, expiry, new SystemClockSupplier());
+  }
+
+  /** Constructor for non-refreshable tokens (e.g. M2M) with ClockSupplier */
+  public Token(
+      String accessToken, String tokenType, LocalDateTime expiry, ClockSupplier clockSupplier) {
+    this(accessToken, tokenType, null, expiry, clockSupplier);
   }
 
   /** Constructor for refreshable tokens. */
   public Token(String accessToken, String tokenType, String refreshToken, LocalDateTime expiry) {
+    this(accessToken, tokenType, refreshToken, expiry, new SystemClockSupplier());
+  }
+
+  /** Constructor for refreshable tokens with ClockSupplier. */
+  public Token(
+      String accessToken,
+      String tokenType,
+      String refreshToken,
+      LocalDateTime expiry,
+      ClockSupplier clockSupplier) {
     Objects.requireNonNull(accessToken, "accessToken must be defined");
     Objects.requireNonNull(tokenType, "tokenType must be defined");
     Objects.requireNonNull(expiry, "expiry must be defined");
+    Objects.requireNonNull(clockSupplier, "clockSupplier must be defined");
     this.accessToken = accessToken;
     this.tokenType = tokenType;
     this.refreshToken = refreshToken;
     this.expiry = expiry;
+    this.clockSupplier = clockSupplier;
   }
 
   public boolean isExpired() {
     if (expiry == null) {
       return false;
     }
-    LocalDateTime potentiallyExpired = expiry.minus(10, ChronoUnit.SECONDS);
-    LocalDateTime now = LocalDateTime.now();
+    // Azure Databricks rejects tokens that expire in 30 seconds or less,
+    // so we refresh the token 40 seconds before it expires.
+    LocalDateTime potentiallyExpired = expiry.minus(40, ChronoUnit.SECONDS);
+    LocalDateTime now = LocalDateTime.now(clockSupplier.getClock());
     return potentiallyExpired.isBefore(now);
   }
 

databricks-sdk-java/src/main/java/com/databricks/sdk/core/utils/ClockSupplier.java

@@ -0,0 +1,7 @@
+package com.databricks.sdk.core.utils;
+
+import java.time.Clock;
+
+public interface ClockSupplier {
+  Clock getClock();
+}

databricks-sdk-java/src/main/java/com/databricks/sdk/core/utils/SystemClockSupplier.java

@@ -0,0 +1,10 @@
+package com.databricks.sdk.core.utils;
+
+import java.time.Clock;
+
+public class SystemClockSupplier implements ClockSupplier {
+  @Override
+  public Clock getClock() {
+    return Clock.systemDefaultZone();
+  }
+}

databricks-sdk-java/src/main/java/com/databricks/sdk/core/utils/RealTimer.java renamed to databricks-sdk-java/src/main/java/com/databricks/sdk/core/utils/SystemTimer.java

@@ -1,6 +1,6 @@
 package com.databricks.sdk.core.utils;
 
-public class RealTimer implements Timer {
+public class SystemTimer implements Timer {
   @Override
   public void wait(int milliseconds) throws InterruptedException {
     Thread.sleep(milliseconds);

databricks-sdk-java/src/test/java/com/databricks/sdk/core/oauth/TokenTest.java

@@ -0,0 +1,74 @@
+package com.databricks.sdk.core.oauth;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import com.databricks.sdk.core.utils.FakeClockSupplier;
+import java.time.Instant;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+import org.junit.jupiter.api.Test;
+
+class TokenTest {
+
+  private static final String accessToken = "testAccessToken";
+  private static final String refreshToken = "testRefreshToken";
+  private static final String tokenType = "testTokenType";
+  private final LocalDateTime currentLocalDateTime;
+  private final FakeClockSupplier fakeClockSupplier;
+
+  TokenTest() {
+    Instant instant = Instant.parse("2023-10-18T12:00:00.00Z");
+    ZoneId zoneId = ZoneId.of("UTC");
+    fakeClockSupplier = new FakeClockSupplier(instant, zoneId);
+    currentLocalDateTime = LocalDateTime.now(fakeClockSupplier.getClock());
+  }
+
+  @Test
+  void createNonRefreshableToken() {
+    Token token =
+        new Token(accessToken, tokenType, currentLocalDateTime.plusMinutes(5), fakeClockSupplier);
+    assertEquals(accessToken, token.getAccessToken());
+    assertEquals(tokenType, token.getTokenType());
+    assertNull(token.getRefreshToken());
+    assertTrue(token.isValid());
+  }
+
+  @Test
+  void createRefreshableToken() {
+    Token token =
+        new Token(
+            accessToken,
+            tokenType,
+            refreshToken,
+            currentLocalDateTime.plusMinutes(5),
+            fakeClockSupplier);
+    assertEquals(accessToken, token.getAccessToken());
+    assertEquals(tokenType, token.getTokenType());
+    assertEquals(refreshToken, token.getRefreshToken());
+    assertTrue(token.isValid());
+  }
+
+  @Test
+  void tokenExpiryMoreThan40Seconds() {
+    Token token =
+        new Token(accessToken, tokenType, currentLocalDateTime.plusSeconds(50), fakeClockSupplier);
+    assertFalse(token.isExpired());
+    assertTrue(token.isValid());
+  }
+
+  @Test
+  void tokenExpiryLessThan40Seconds() {
+    Token token =
+        new Token(accessToken, tokenType, currentLocalDateTime.plusSeconds(30), fakeClockSupplier);
+    assertTrue(token.isExpired());
+    assertFalse(token.isValid());
+  }
+
+  @Test
+  void expiredToken() {
+    Token token =
+        new Token(accessToken, tokenType, currentLocalDateTime.minusSeconds(10), fakeClockSupplier);
+    assertTrue(token.isExpired());
+    assertFalse(token.isValid());
+  }
+}

databricks-sdk-java/src/test/java/com/databricks/sdk/core/utils/FakeClockSupplier.java

@@ -0,0 +1,18 @@
+package com.databricks.sdk.core.utils;
+
+import java.time.Clock;
+import java.time.Instant;
+import java.time.ZoneId;
+
+public class FakeClockSupplier implements ClockSupplier {
+  private final Clock clock;
+
+  public FakeClockSupplier(Instant fixedInstant, ZoneId zoneId) {
+    clock = Clock.fixed(fixedInstant, zoneId);
+  }
+
+  @Override
+  public Clock getClock() {
+    return clock;
+  }
+}