task a8: move all binaries under ./cmd/<name>/main.go

Relocates the two non-canonical main packages so every binary in the repo
lives at ./cmd/<BINARY>/main.go:

- tools/filewriter/ -> cmd/filewriter/
- integrationtests/cmd/ioworkload/ (20 files) -> cmd/ioworkload/

Consumers updated:
- Magefile.go: workloadSourcePath now ./cmd/ioworkload
- integrationtests/README.md: structure note points at ../cmd/ioworkload

Files moved with git mv so git log --follow history is preserved.
cmd/ior/main.go was already canonical and is untouched.

Verified: mage build produces the ior binary; go build ./cmd/...
builds filewriter and ioworkload; go test ./cmd/ioworkload passes;
go vet ./cmd/filewriter ./cmd/ioworkload is clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

1 files changed, 93 insertions, 0 deletions

diff --git a/cmd/ioworkload/scenario_truncate.go b/cmd/ioworkload/scenario_truncate.go
new file mode 100644
index 0000000..04288d5
--- /dev/null
+++ b/cmd/ioworkload/scenario_truncate.go
@@ -0,0 +1,93 @@
+package main
+
+import (
+	"fmt"
+	"path/filepath"
+	"runtime"
+	"syscall"
+	"unsafe"
+)
+
+// truncateBasic opens a file, writes data, then truncates it via
+// syscall.Truncate which uses SYS_TRUNCATE directly on amd64 (path-based).
+func truncateBasic() error {
+	dir, cleanup, err := makeTempDir("truncate-basic")
+	if err != nil {
+		return err
+	}
+	defer cleanup()
+
+	path := filepath.Join(dir, "truncfile.txt")
+	fd, err := syscall.Open(path, syscall.O_RDWR|syscall.O_CREAT, 0o644)
+	if err != nil {
+		return fmt.Errorf("open: %w", err)
+	}
+
+	if _, err := syscall.Write(fd, []byte("truncate this content")); err != nil {
+		syscall.Close(fd)
+		return fmt.Errorf("write: %w", err)
+	}
+	syscall.Close(fd)
+
+	return syscall.Truncate(path, 5)
+}
+
+// truncateFtruncate opens a file, writes data, then truncates it via
+// syscall.Ftruncate which uses SYS_FTRUNCATE directly on amd64 (fd-based).
+func truncateFtruncate() error {
+	dir, cleanup, err := makeTempDir("truncate-ftruncate")
+	if err != nil {
+		return err
+	}
+	defer cleanup()
+
+	path := filepath.Join(dir, "ftruncfile.txt")
+	fd, err := syscall.Open(path, syscall.O_RDWR|syscall.O_CREAT, 0o644)
+	if err != nil {
+		return fmt.Errorf("open: %w", err)
+	}
+	defer syscall.Close(fd)
+
+	if _, err := syscall.Write(fd, []byte("ftruncate this content")); err != nil {
+		return fmt.Errorf("write: %w", err)
+	}
+	return syscall.Ftruncate(fd, 5)
+}
+
+// truncateEnoent attempts to truncate a nonexistent file via raw SYS_TRUNCATE.
+// The syscall fails with ENOENT, but ior captures the enter_truncate
+// tracepoint because the path is read on entry.
+func truncateEnoent() error {
+	dir, cleanup, err := makeTempDir("truncate-enoent")
+	if err != nil {
+		return err
+	}
+	defer cleanup()
+
+	path := filepath.Join(dir, "truncate-enoent-missing.txt")
+	pathBytes, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return fmt.Errorf("path bytes: %w", err)
+	}
+	// Retry a few times to make this test resilient under high integration
+	// parallelism where a single failed syscall event can be dropped.
+	for i := 0; i < 5; i++ {
+		_, _, errno := syscall.Syscall(syscall.SYS_TRUNCATE, uintptr(unsafe.Pointer(pathBytes)), 0, 0)
+		runtime.KeepAlive(pathBytes)
+		if errno == 0 {
+			return fmt.Errorf("expected ENOENT, but truncate succeeded")
+		}
+	}
+	return nil
+}
+
+// truncateFtruncateEbadf calls raw SYS_FTRUNCATE on an invalid fd (99999).
+// The syscall fails with EBADF, but ior captures the enter_ftruncate
+// tracepoint because it is recorded on syscall entry.
+func truncateFtruncateEbadf() error {
+	_, _, errno := syscall.Syscall(syscall.SYS_FTRUNCATE, 99999, 0, 0)
+	if errno == 0 {
+		return fmt.Errorf("expected EBADF, but ftruncate succeeded")
+	}
+	return nil
+}