test(port_arm): buffer the panic message

Fixes the test runner only displaying the very first part of the panic
message as shown below:

    Test run 'kernel_tests::compute_round_robin' failed: 'panicked at ''
    Failed tests:
     - kernel_tests::compute_round_robin
    Command failed.

Author: yvt

Cargo.lock

@@ -73,7 +73,7 @@ impl MemoryMapSection {
     ///  - `physical_start` is of type `u64`, but using a large physical address
     ///    (> 4GiB) isn't supported yet.
     ///
-    /// The memory section is configured as a read/writable (bot not
+    /// The memory section is configured as a read/writable (but not
     /// executable) Normal memory with a Outer and Inner Write-Back,
     /// Write-Allocate attribute.
     ///

src/constance_port_arm/src/startup/cfg.rs

@@ -27,6 +27,7 @@ board-rza1 = [
 
 output-semihosting = [
     "arm_semihosting",
+    "staticvec",
 ]
 
 [dependencies]
@@ -35,6 +36,7 @@ constance_port_arm = { path = "../constance_port_arm", optional = true }
 arm_semihosting = { path = "../arm_semihosting", optional = true }
 constance = { path = "../constance", optional = true }
 
+staticvec = { version = "0.10.2", optional = true, default-features = false }
 register = { version = "0.5.1", optional = true }
 log = { version = "0.4.8", optional = true }
 

src/constance_port_arm_test_driver/Cargo.toml

@@ -1,13 +1,29 @@
 use arm_semihosting::{debug, debug::EXIT_FAILURE, hio};
 use core::{fmt::Write, panic::PanicInfo};
+use staticvec::StaticString;
+
+static mut BUFFER: StaticString<{ 512 }> = StaticString::new();
 
 #[panic_handler]
 fn panic(info: &PanicInfo) -> ! {
     // Disable interrupts
     unsafe { llvm_asm!("cpsid i"::::"volatile") };
 
     if let Ok(mut hstdout) = hio::hstdout() {
-        writeln!(hstdout, "{}", info).ok();
+        // The test runner stops reading the output when it encounters a stop
+        // word (`panicked at`). Actually it continues reading for some time,
+        // but semihosting output incurs a huge delay on each call and the
+        // `Display` implementation of `PanicInfo` produces a message in small
+        // chunks, so the test runner would stop reading after the first chunk
+        // (`panicked at '`).
+        //
+        // To avoid this problem, put the whole message in a buffer and send it
+        // with a single semihosting call.
+        let buffer = unsafe { &mut BUFFER };
+        buffer.clear();
+        let _ = writeln!(buffer, "{}", info);
+
+        let _ = write!(hstdout, "{}", buffer);
     }
     debug::exit(EXIT_FAILURE);