Fix not to raise an exception from `(Service|Client)RequestContext.cu… (

line#2962)

* Fix not to raise an exception from `(Service|Client)RequestContext.currentOrNull()`
Motivation:
`ServiceRequestContext.currentOrNull()` raises an `IllegalArgumentException`
if the current ctx is `ClientRequestContext` and `ctx.root()` returns `null`.

It's very confusing because all other APIs returns `null` if the name ends with `orNull`.
So I think the API should return `null` instead of raising an exception.
We could use `ClientRequestContext.roo()` to find out if the client call is invoked by a server request or not.

Modification:
- Returns `null` instread of raising an exception from `(Service|Client)RequestContext.currentOrNull()`

Result:
- `(Service|Client)RequestContext.currentOrNull()` does not throw an exception anymore.

* Address the comment by @trustin

core/src/main/java/com/linecorp/armeria/client/ClientRequestContext.java

@@ -74,17 +74,14 @@ static ClientRequestContext current() {
      * Returns the client-side context of the {@link Request} that is being handled in the current thread.
      *
      * @return the {@link ClientRequestContext} available in the current thread, or {@code null} if unavailable.
-     * @throws IllegalStateException if the current context is not a {@link ClientRequestContext}.
      */
     @Nullable
     static ClientRequestContext currentOrNull() {
         final RequestContext ctx = RequestContext.currentOrNull();
-        if (ctx == null) {
-            return null;
+        if (ctx instanceof ClientRequestContext) {
+            return (ClientRequestContext) ctx;
         }
-        checkState(ctx instanceof ClientRequestContext,
-                   "The current context is not a client-side context: %s", ctx);
-        return (ClientRequestContext) ctx;
+        return null;
     }
 
     /**
@@ -105,6 +102,12 @@ static <T> T mapCurrent(
             return mapper.apply(ctx);
         }
 
+        final ServiceRequestContext serviceRequestContext = ServiceRequestContext.currentOrNull();
+        if (serviceRequestContext != null) {
+            throw new IllegalStateException("The current context is not a client-side context: " +
+                                            serviceRequestContext);
+        }
+
         if (defaultValueSupplier != null) {
             return defaultValueSupplier.get();
         }

core/src/main/java/com/linecorp/armeria/server/ServiceRequestContext.java

@@ -91,14 +91,7 @@ static ServiceRequestContext currentOrNull() {
             return null;
         }
 
-        final ServiceRequestContext root = ctx.root();
-        if (root != null) {
-            return root;
-        }
-
-        throw new IllegalStateException(
-                "The current context is not a server-side context and does not have a root " +
-                "which means that the context is not invoked by a server request. ctx: " + ctx);
+        return ctx.root();
     }
 
     /**
@@ -119,6 +112,14 @@ static <T> T mapCurrent(
             return mapper.apply(ctx);
         }
 
+        final ClientRequestContext clientRequestContext = ClientRequestContext.currentOrNull();
+        if (clientRequestContext != null) {
+            throw new IllegalStateException(
+                    "The current context is not a server-side context and does not have a root " +
+                    "which means that the context is not invoked by a server request. ctx: " +
+                    clientRequestContext);
+        }
+
         if (defaultValueSupplier != null) {
             return defaultValueSupplier.get();
         }

core/src/test/java/com/linecorp/armeria/client/ClientRequestContextTest.java

@@ -62,9 +62,7 @@ void currentOrNull() {
         assertCurrentCtx(null);
 
         try (SafeCloseable unused = serviceRequestContext().push()) {
-            assertThatThrownBy(ClientRequestContext::currentOrNull)
-                    .isInstanceOf(IllegalStateException.class)
-                    .hasMessageContaining("not a client-side context");
+            assertThat(ClientRequestContext.currentOrNull()).isNull();
         }
     }
 

core/src/test/java/com/linecorp/armeria/server/ServiceRequestContextTest.java

@@ -75,9 +75,7 @@ void currentOrNull() {
         assertCurrentCtx(null);
 
         try (SafeCloseable unused = clientRequestContext().push()) {
-            assertThatThrownBy(ServiceRequestContext::currentOrNull)
-                    .isInstanceOf(IllegalStateException.class)
-                    .hasMessageContaining("not a server-side context");
+            assertThat(ServiceRequestContext.currentOrNull()).isNull();
         }
     }